Evolution & Systematics

The group Hemichordata is known for being a connection between theinvertebrates and the chordates since it has embryological features similar toechinoderms and has chordate’s characteristics in the adult stage (Hyman 1959). Due to Hemichordatemorphological features, the phylogenetic tree regarding deuterostomes is stilldebatable (Gee 1996; Castresana et al 1998),however molecular experiments point out that hemichordates are moreintimately related to echinoderms rather than chordates (EncyclopaediaBritannica, undated). Therefore, this group is an importantbasis of scientific knowledge about the origin of chordates andthe bilateral body plan, (Mertz 2004).

Compared to Chordate, both of them havepharyngeal gills (Mandal, 2012). Moreover, Hemichordata has a structure similar to thenotochord that is a hollow collar cord (Mandal,2012). Finally, they have M-shaped skeletal rods and synapticula that isprecisely equal the ones of Amphioxous (Mandal, 2012). However, Chordates do not have sections forthe body and coelom like Hemichordata has (Mandal,2012).

Considering echinoderms, they have around five similarities. First, the tornaria larvae (Hemichordata) and bipinnaria(Echinodermata) are very similar, both with ciliated band in the body (Mandal, 2012)(figre 14). Furthermore, their digestive tracts havepretty much the same shape and divisions (Mandal,2012). In both cases, the blastopore turns into larval anus (Mandal, 2012). Moreover, cleavage and gastrulation showsidentical patterns (Mandal, 2012). Finally, coelomin its origin is enterocoelous (Mandal, 2012).Yet, they present some dissimilarities: in the echinoderms larvae the eyespot,apical plate and telotroch are absent and protocoel is paired while inHemchordata it is unpaired (Mandal, 2012).

Figure 14 - Comparison of a hemichordate tornaria (A) to an echinoderm bipinnaria (B) (Encyclopaedia Britannica, undated)

The Hemichordata still have features that canbe compared with other animals' groups, such as Annelida, Phoronida andPogonophora.

The similarities between Hemichordata andAnnelida are: overall body characteristics, burrowing habitat and vascularsystem with blood flowing interiorly in dorsal vessel and posteriorly in theventral vessels (Mandal, 2012). Though,annelids do not have gills slits and have paired nerve cord. These differencesare very noticeable, thus they do not have phylogenetic relationship (Mandal, 2012).

With Phoronida they share the disposition ofthe coelom, presence of a ring of cilia surrounding the anus and presence of comparablepore, in the proboscis in Hemichordata and a water pore in Actinotrocha (Mandal, 2012).

Finally, with Pogonophora is possible to noticesimilarities like the partition of enterocoelous coelom, presence ofintraepidermal nervous system, presence of gonads in the trunk section andpresence of septum between mesosome and metasome (Mandal, 2012). However, their main nervous system differs;it is concentrated in protosome while in Balanoglossus it is concentrated atmesosome (Mandal, 2012). Moreover, Pogonophoredoes not have some alimentary canal (Mandal, 2012). These similarfeatures are probably because of its commom phhylogenetic connection (Mandal,2012).