TYPE MOLLUSCA. 285 



nerve-cells into concrete ganglia ; they are scattered along the 

 longitudinal nerve-cords, of which there are two pairs, i.e., the 

 pleuro-visceral cords, which run along the lateral portions of 

 the body, and the pedal cords, which are situated more 

 ventrally and which, as well as the pleuro-visceral, are fre- 

 quently united by cross-commissures which suggest an imper- 

 fect metamerism. In front these cords unite together to form 

 the circuinoasophageal ring in which the ganglion-cells are 

 somewhat more numerous than elsewhere, without, however, 

 forming distinct ganglia. Sense-organs are but slightly de- 

 veloped throughout the group, which is divisible into two well- 

 marked orders. 



1. Order Solenogastres. 



The members of this order are for the most part elongated 

 worm-like animals, though some forms are short (Fig. 126) 

 and more nearly approach the typical 

 Molluscan form. The exterior of the 

 body is covered by an exceptionally 

 thick cuticle traversed by bands of 

 cells extending into it from the ecto- FlG 12G ^Neomenia ca- 

 dernial layer of the body and terrnina- rinata (after NANSEN). 

 ting in cup-shaped groups of cells c i _ cteuidium. 

 which secrete the calcareous spicules m = mouth. 

 which are scattered through the cuti- ^ = ventral groove. 

 cle (Proneomenia) or may project upon 



its surface (Chcetoderma), and which are the sole repre- 

 sentatives of the shell of the higher Mollusca. Upon the 

 ventral surface of the body is a longitudinal furrow (Fig. 126, 

 vg) at the bottom of which lies the but slightly developed 

 foot, represented by a small ciliated longitudinal ridge, which in 

 CJwetoderma may be quite undeveloped, the furrow being in this 

 form also barely indicated or entirely absent. The lips of the 

 furrow which enclose the foot probably represent the mantle- 

 folds of higher forms, here very much reduced, though more 

 extensively developed at the posterior end of the body, where 

 they project to form a funnel-like structure (Fig. 127) whose 

 cavity the cloaca receives the openings of the digestive 



