After fertilisation, the eggs are brooded in the mantle cavity until they develop into planktotrophic naupliar larvae. The Nauplius is a very different shape to the adult barnacle, it has an outer shell (carapace)with two projected horns at the front and one rear protective spine. The larvae has a single eye located on the front dorsal surface, as well as a total of three paired limbs with long setae attached to them (Figure 6 & 7). (Poore &Syme, 2009) The development in the Nauplius is tightly constrained to 6 stages.(West & Costlow, 1988) As the Nauplius progresses through each moult stage the general form and functions of body larval parts remains unchanged. Naupliar limbs are used for swimming, collection of food and ingestion. All naupliar stages except the initial larva feed by filtering particles from plankton. The Nauplius also responds positively to light and stays close to the surfaces, which avoids being caught by filter feeders on the bottom. (Poore & Syme, 2009)

Bodily functions and proportions are all relatively similar across all stages of development with the major exception of the thoraco-abdominal process which becomes larger at each stage of naupliar development. This enlargement can also provide a way of determining the stage of development in the naupliar larvae. Another major change seen during development is an increase in the number of setae, variety of setae and overall increasing complexity found in the naupliar limbs through progressing molts as seen in figure 6. (Anderson, 1994; West & Costlow, 1988)

All pairs of limbs are used in swimming, but for food collection each pair has differing functions. Antennules are not used at all in food collection and the antennae are the major contributor to the collection of food particles. The mandibles are responsible for the transfer of food particles from the antennae to the mouth.(Walker, Yule & Nott, 1987) In locomotion the antennae are the main swimming limbs. They swing through a wide arc along the sides of the naupliar body. The setae are spread on the pull stroke and feathered during the recovery. Forward movement in the Nauplius is very jerky. The mandibles and antennules beat in a metachronal rhythm with the antennae but only provide minor assistance in propulsion. (Anderson, 1994; Walker, Yule & Nott, 1987) 

The planktotrophic stage VI Nauplius moults into a bivalve non-feeding cyprid. The cyprid has the ability to seek out, identify and attach to a substrate appropriate for adult life. (Anderson, 1994) Pre-settlement metamorphosis from Nauplius to cyprid is functionally and structurally different. Naupliar feeding and swimming features are lost. With the protective dorsal shield extending as a bivalve carapace which completely encloses the body. Mantle cavity formation occurs just under the carapace at both the anterior and posterior ends. The carapace loses frontolateral horns and shield spines found in the naupliar stage and is now instead anteriorly rounded with the posterior end tapering to a blunt point.(Walker, Yule & Nott. 1987) The carapace can be opened or closed via the use of the adductor muscle. When open, a pair of antennules can be extended from the anterior mantle cavity and six pairs of biramous thoracic limbs along with a pair of uniramous caudal appendages from the posterior cavity. (Figure 8) (Walker& Lee, 1976)

Swimming form differs significantly between the cyprid stage and the Nauplius stage. The cyprids thoracic limbs provide propulsion in a forwards but also upwards motion. The behaviour of the larva requires it to swim at a fixed depth to optimise the chance of finding a suitable location to settle. This means that the cyprid must pause between movement to sink down in the water column before moving again. (Yule, 1982) The cyprid senses its orientation in the water column with the use of mainly light receptors and pressure. Locomotion in the cyprid stage is increasingly important because the main role of the cyprid is to find a suitable substrate to settle. (Anderson, 1994)

The cirri are the feeding structure found in most barnacles.They are a modified form of the thoracic appendages found in the cyrpid larval stage, the cirri range from I (1) to VI (6). Cirri I to III are relatively smaller, specialised maxillipeds. Cirri IV to VI are all long and fairly similar in structure, and are the main cirri used for capturing zooplankton,tiny phytoplankton cells or bacteria in their setae. (Figure 9) (Anderson, 1994; Poore & Syme, 2009) Cirri I to III transfer food particles capture by longer cirri into the oral cone. The motion of food transfer between the longer cirri and the maxillapeds creates a ‘beating’ motion which pushes water into the mantle cavity. This helps respiration but also potentially catches any extra food particles not caught by the longer cirri. T. rosea is dependent upon strong water movement for cirral activity, and has more rapid actions of the cirri and operculum. (Anderson,1994)

Circulation in T. rosea involves the controlled flow of haemocoelic fluid through the haemocoel. The haemocoel is a fluid skeleton used for movements and circulation in barnacles. It has replaced muscles as means of extending structures such as valves, thorax, cirri and penis. (Anderson, 1994) The mantle tissue lining the mantle cavity is used as a compressible reservoir,from this blood is pushed into the mantle tissue lining the opercular valves causing them to part. Outflow from the mantle is diverted into paired branchiae which inflate when the operculum is opened. Scutal vessels lead by a scutal valve enter into various parts of the prosoma including the cirri, oral cone, thorax and penis. Collecting circulation is controlled by a large rostral sinus and valve regulating activity and withdrawal. The rostral sinus pumps the haemolymph back to the mantle to be recirculated. (Burnett, 1977; Anderson,1994)

Respiration in T.rosea occurs via the surface of the body, limbs and lining surfaces present in the mantle cavity. Haemocoelic circulation is continuously supplied to these areas for respiratory gas exchange. The main respiratory organ in T. rosea and most barnacles the branchiae, it is a paired highly vascularised extension of the mantle wall. (Figure 11) The walls of the branchiae are folded to increase surface area. This structure is ventilated by water flow through the rhythmic beating motion produced by the cirri. There is some evidence that increased ventilatory action in the cirri to maximise respiratory exchange is observed when an increased CO₂ concentration is experienced. (Anderson and Southward, 1987)

Intertidal barnacles show a special adaptation in relation to respiratory exchange when exposed and emersed to air. When exposed out of water the barnacle expels water from the mantle cavity and replaces it with an air bubble. If conditions causing desiccation are not present the operculum is left slightly open and serves as a micropyle. The air bubble held inside the mantle cavity facilitates gas exchange between branchiae, lining of the mantle and the external air. If desiccation becomes an issue, the operculum is closed and respiration becomes anaerobic. (Grainger and Newell, 1965) 

Barnacles have discrete excretory organs called the maxillary glands which are located in the prosoma either side of the foregut. It is theorised that the adult maxillary glands are the route for nitrogenous excretion. (Figure 12) (Anderson, 1994) Each maxillary gland has a small end sac inside a haemocoelic sinus, the end sac links to efferent duct via a specialised funnel formed by 4 modified end sac cells.  The efferent duct is the largest part of the maxillary gland, the true purpose of the duct is not currently known however it is thought to be a storage bladder for urine. (White & Walker, 1981) The scutal sinus keeps the end sac highly saturated with haemolymph whereas the efferent duct is more restricted. Overall little is known about the function of the maxilliary glands as excretory organs in barnacles. (Anderson, 1994)

T. rosea, as with many other barnacles has a very simplified nervous system. There is a single ventral ganglionic chain fused into one mass which is connected to the supra-oesophageal ganglion. From these ganglion nerves branch out to muscles and structures such as the cirri, penis and thorax. (Anderson, 1994) Barnacles have lost many of the sensory organs found in the larval stages. However they do respond to light queues when cirri are unfurled. This is called the shadow reflex and is a defensive mechanism causing the barnacle to quickly retract its cirri and close the operculum. (Anderson, 1994; Gwilliam, 1963; Gwilliam, 1965)Other sensory structures in barnacles include chemoreceptors which are found on opercular valves, setae in the penis and on the cirri. (Anderson, 1994)