INVERTEBRATE ZOOLOGY. 13 



Type V. Ambulacralia (with one Cladiis). 

 Cladus. Echinodermata. 



Primarily bilateral pelagic forms, which attain a secondary radiate penta- 

 merous structure, after which they may become creeping or even sessile. The 

 bilateral larvae are all derivable from a common form, the Pro-auriculavia, 

 which is itself derived directly from the gastraea by the formation of a sec- 

 ondary month and the conversion of the protostoma into the anas. This form 

 somewhat resembles the trochozocin, but differs from it in the origin of mouth 

 and anus, and in the absence of an apical sense organ. 



During development the protocoelom becomes filled with mesenchyma. and 

 the definite coelom appears in the form of a pair of mesodermic sacs (or one 

 double sac), the eiiterocoeles, from which arise typically a pair of secondary 

 sacs, assiiming the form of five-pointed rosettes, the hydrocoeles. Of these, 

 the right one aborts and the left one grows into the water-vascular system of 

 the definite radiate form. In most cases the right hydrocoele is entirely sup- 

 pressed. The adult form possesses oral and ab-oral surfaces, which corres- 

 pond respectively to the left and right sides of the posterior portion of the larva. 



Larval locomotion is by cilia, the adult mainly by the water-vascular sys- 

 tem. Characteristic in all stages is the possession of a skeleton, composed of 

 calcareous elements, which may form closely articulated plates, a network of 

 loosely woven spicules, or may be reduced to minute graniiles imbedded in the 

 integument. 



[P. and L. Sarasin and others attribute to the hydrocoeles the value of nephridia, and 

 Bury claims that the enterocoele normally develops from tiro pairs of coelom-sacs.] 



Type VI. Chord ata. 



Bilateral animals with dorsal nervoTis system, supported ventrally during 

 some stage at least by a skeletal rod. the notochoi-d. derived from the endo- 

 derm. The pharynx is perforated by lateral slits, respiratory in function, 

 which may develop gills, supported by skeletal rods, the visceral or gill arches. 

 The protostoma is embryonic and occupies the future dorsal region, appearing 

 in higher euibryt)es as an elongated groove or cleft, the primitive streak. The 

 mesoderm arises from the lateral region of the primordial intestine, typically 

 in the form of paired sacs, segmentally arranged, but in modified forms as 

 more or less solid masses, which segment and develop cavities after separation 

 from the endoderm. From these mesodermic cavities the metacoelom is 

 formed. The protocoelom becomes filled with mesenchyma, which forms the 

 connective tissues and the blood vessels. 



