14 INVERTEBRATE ZOOLOGY. 



Cladus I. Hemichorda. 



Notochord confined to anterior part of body above pharynx. Nervous sys- 

 tem dorsal and ventral chords, not entirely distinct from ectoderm. Pharynx 

 perforated by paired gill slits. Genital organs consist of several pairs of 

 sac-gonads. opening independently on the dorsal side. As diverticula of the 

 primordial intestine {archenteron) arise 2-3 pairs of coelom-sacs, as well as a 

 median dorsal jjore. compared by some to the dorsal pore of Echinoderm lar- 

 vae. This appears double in some species. Some species develop directly, 

 others by a free-sv^imming larva, the Tornaria, which appears in some re- 

 spects similar to Proauricularia. and in others to the trochophore. Develop- 

 ment by Tornaria-larva, is undoubtedly more primitive than the direct method, 

 and hence Tornaria must represent the free-swimming pelagic ancestor of the 

 Chordata. Its relationship to the other primitive forms is uncertain. 



Cladus II. Urochorda {Tiinicata). 



A group of marine Chordata, representing various stages of degeneracy, 

 and thus of very varied external appearance. The larvae are free-swimming 

 and pelagic, but may become sessile in adult life. The notochord is confined 

 to the caudal region where it supports the posterior part of the dorsal nervous 

 system. It is persistant throughout life in a single group of small pelagic 

 forms, the Perennichordata (Appendiciilaria), becoming rediTced in all others. 

 Corresj)onding to the changes in the notochord, the larval nervous system is 

 elongated, but in the adult becomes reduced to a single ganglion, Ij'ing dorsal 

 to the pharynx. The mesoderm arises as in typical chordata. but is without 

 trace of segmentation, and lacks the coelomic cavities, which either do not 

 appear or become early obliterated. Nephridia fail. The body of the adi;lt 

 is enclosed in a mantle or tunic, which may be gelatinous, leathery or carti- 

 laginous, and posesses two orifices ; an incurrent or oral, and an excurrent or 

 cloacal. The pharynx is perforated by gill slits which may appear in two 

 lateral rows, or by secondary reduplication may convert the entire pharyngeal 

 wall into a lattice- work. The water passes from these slits into a peri- 

 branchial or cloacal cavity, formed by the mantle, and into which the intes- 

 tine and the genital ducts also open. In the sessile forms the intestine is 

 U-shaped, and the two mantle openings approximate, while in the free-swim- 

 ming forms they are situated at the two poles of the oval or cask-shaped bod}^ 

 Asexual reproduction combined with complicated alternation of generations 

 occur in this group. The sexual forms are hermaphroditic. 



