12 INVERTEBRATE ZOOLOGY. 



Sub-cladus I. Annelida. 



Appendages in the form of parapodia, i. e. fleshy outgrowths of the lateral 

 walls, bearing bristles, gills, and other organs, but never jointed. In some 

 forms the processes are suppressed, the bristles being inserted in pits. No 

 marked differentiation of the body into definite body regions. 



In this group appear the most typical Trochophore larvae. 



Sub-cladus II. Arthropoda. 



Appendages jointed. A Trochophore larva does not appear, but their posi- 

 tion is determined by means of their marked relationship to the Annelids. 

 Development by larval forms is common, but many of these are plainly sec- 

 ondarily acquired and of no value in phylogenesis. 



Cladus II. Mollusca. 



Metasoma consisting of one segment, which early fuses with the prosoma, 

 resulting in an apparently unsegmented body. Metanephridia a single pair. 

 Metacoelom almost filled with parenchymatous tissue, the pericardial cavity 

 and those of the germ glands alone remaining. The body consists typically 

 of head, foot and visceral sack, of which the first two may fail or be modified 

 beyond recognition. The body is generally surrounded by a duplicature of 

 the integument termed the mantle, which secretes a calcareous shell and en- 

 closes a branchial cavity. As reminiscence of the Trochozoon, there fre- 

 quently appears the " Veliger " larva, similar to the Trochophore. 



Cladus III. MoUuscoidea. 



Sessile animals, mostly marine and often colonial, either enclosed in cells or 

 protected by dorsal and ventral calcareous shells. Mouth surrounded by a 

 ridge, the lophophore, which bears ciliated tentacles, or two spiral ciliated 

 arms. Intestine U-shaped, with anus just outside the lophophore. Meta- 

 coelom present, developing in the typical forms from a pair of lateral sacs. 

 One pair of metanephridia. Central nervous system a ganglion between 

 mouth and anus. The apparent lack of segmentation may be explained here 

 as in Mollusca. by supposing a simple metasoma 



