THE PHYLA HEMICHORDATA AND CHORDATA 



of the proboscis, arising as an anterior extension from the dorsal wall of the 

 buccal cavity. This small supporting element is probably homologous with 

 the chordate notochord, but it is limited in its extent and of a very primitive 

 nature. 



The sexes are separate in the balanoglossids; the gonads are sac-like 

 structures arranged serially on each side in the region of the genital ridges. 

 When mature, each gonad establishes an individual, externally opening geni- 

 tal pore. The small eggs are fertilized externally, and development proceeds 

 in the sea water. A pelagic, ciliated larva, the tornaria (Fig. 18.1), occurs as a 

 developmiental stage in the life cycles of some species, but not in Saccoglossus. 

 The tornaria is so similar to the dipleurula-type larva of echinoderms that it 

 was originally described as the larva of .some unidentified echinoderm. This 

 similarity may be interpreted as indicating an evolutionary relationship be- 

 tween echinoderms and hemichords, a relationship which is suggested also on 

 the basis of certain biochemical similarities. 



The Pterobranchia: Cephalodiscus and Rhabdopleura. These are 

 small marine forms with the typical hemichord body regions of proboscis, 

 collar, and trunk (Fig. 18.2); the internal anatomy is comparable with that 

 of the balanoglossids. In correlation with their generally sedentary mode of 

 life, the gut is U-shaped, the anus opening near the mouth. The proboscis 

 is modified into two or many arms, or tentacles, which in Rhabdopleura are 

 retractable into the secreted test within which the animal lives. Reproduction 

 is both sexual and asexual; in Rhabdopleura new individuals are produced by 

 budding from a horizontal stolon growing along the substrate. Individuals 

 thus formed constitute a colony of zooids comparable with those of colonial 

 coelenterates. The fossil remains of the extinct graptolites, long considered 

 as either coelenterates or ectoprocts, have in the light of recent studies 

 been interpreted as showing many similarities to the sessile and enclosed 

 pterobranchs. 



Relationships of the Hemichordata. In the phyla Echinodermata, 

 Chaetognatha (p. 365), Hemichordata, and Chordata, the embryonic origin 

 of the coelom is by the enterocoelous method, as in no other animal groups 

 except the anomalous Brachiopoda (pp. 361-365). This common embryo- 

 logical feature is considered to indicate that these four groups are related by 

 a common ancestry. Evidence from serological studies (see page 635) shows 

 that the proteins of chordates resemble those of echinoderms and hemichords 

 more closely than those of any other invertebrate group. Further, biochemi- 

 cal studies on substances termed phosphagens, important in the functions of 

 muscular tissues, also reveal significant similarities between these three groups. 

 The tornaria larva is suggestive of an evolutionary relationship between 

 echinoderms and hemichords, and the presence in hemichords of visceral 

 clefts, the stomochord, and rudiments of a dorsal nerve cord suggests affinities 

 with the chordates. On the whole, the evidence may logically be interpreted 

 as supporting the concept of a common evolutionary line of enterocoelous, 

 deuterostomous organisms, possibly resembling the dipleurula or tornaria, 



543 



