INVERTEBRATE ZOOLOGY. 45 



Sub-class I. Palechinida Shell with more than twenty rows of 



plates — all fossil. 



Snb-class II. El'chinida Shell with twenty rows of plates, ten 



ambulacral and ten interambulacral. 



Order 1. Regularia. . : Mouth and anus in the center of their 



respective' surfaces. 

 -^ Strongyloeentrotus. 



Order 2. Clypeastroidea Mouth central, anus eccentric. 



■^ Echinarachnins. 



Order 3. Spatangoidea Mouth and anus both eccentric. 



■if Spatangns. 



Type VI. Chordata. 



Cladus I. Hemichorda. 



This Cladus was founded for a single genus of worm-like marine forms, 

 found in mud-flats. At the anterior end is a long, flexible proboscis, with 

 which the animal pushes its way through the mud : at the base of this is a 

 naiTow zone, the collar, at the upper ventral edge of which the mouth is situ- 

 ated. This is followed by a long worm -like body, showing paired gill-slits on 

 its ventral aspect. The main details of its structure have been given under 

 the description of the Cladus. q. v. if Bahnwglossiis. 



[Cejjhalodiscns and Rhabdopleura, two sessile forms classed as Order, Pterobranchia, un- 

 der Bryozoa, have been found to resemble Balanoglossus. In young buds of the former a 

 division into proboscis, collar and body may be seen. There are also a single pair of gill- 

 slits, and a dorsal diverticulum of the intestine (— notochord ?) lying under the dorsal ner- 

 vous system. In Rhahdoplema no gill-slits have been detected, but in other respects the 

 structure is similar to Cephalodiscuit. Hemichorda may thus be represented by two 

 classes: Class I, Enteropneusta, including the different species of Bnlanoglossna, and 

 Class II, Pterobranchia, including the two forms under consiaeration.] 



Cladus II. Urochorda. 



Class I. Perennichord.jlTa Free-swimming forms, like the larvae of 



higher tunicates. They possess a long 

 tail provided with a skeletal axis, the 

 notochord. Pharynx with a single pair 

 of gill-.slits. No definite mantle, but a 

 gelatinous envelope, if Appendicularia. 



