474 THE INVERTEBRATA 



figure shows, however, organs which are not present in the annehd. 

 On the dorsal side between the prototroch and the anus the larval 

 ectodermal epithelium forms the rudiment of the mantle and even at this 

 early age secretes the first shell. On the ventral side, there is a promi- 

 nence which is t\iQ foot (formed by the union of two rudiments). The 

 single mesoderm cell gives rise first of all to two regular mesoderm bands ; 

 and by the development of a cavity in each of these, right and left 

 coelomic sacs are formed ; then instead of segmenting as in the annelid, 

 these largely break up into single cells, some elongating and becoming 

 muscle cells (Fig. 350 C). It is because there is never any commence- 

 ment of segmentation in the embryonic mesoderm in molluscs that 

 we have the strongest grounds for believing that molluscs never had 

 segmented ancestors. The trochosphere is followed by a second free- 

 swimming stage, the veliger (Fig. 351), in which the prototroch de- 

 velops, with the postoral ciliated ring or metatroch, into an organ, the 

 velum ^ of increased importance, which serves not only for locomotion 

 but also for feeding, the cilia creating a current which brings particles 

 into the mouth. In the veliger stage the foot increases in size and the 

 shell often becomes coiled in the Gasteropoda. 



Class AMPHINEURA 

 MoUusca with an elongated, bilaterally symmetrical body, the mouth 

 and anus at opposite ends; with a head, without tentacles or eyes, 

 tucked under the mantle^ which occupies the whole of the dorsal 

 surface, and contains various kinds of calcareous spicules imbedded 

 in cuticle, sometimes united to form continuous shells; a flattened 

 foot sometimes reduced; a nervous system (Fig. 373) without definite 

 ganglia, the ganglion cells being evenly distributed along the length of 

 the nerve cords, and composed of a circumoesophageal commissure 

 and two pairs of longitudinal cords {pedal and palliovisceral)^ each 

 pair united by a posterior commissure dorsal to the rectum ; a radula ; 

 usually a trochosphere larva. 



PoLYPLACOPHORA. Amphineura with flat foot which occupies the 

 whole ventral face of the body; mantle containing eight transverse 

 calcareous plates as well as spicules; in the mantle groove which runs 

 entirely round the body there is a more or less complete row of 

 ctenidium-like gills on each side, e.g. Chiton (Figs. 349 B, 352), 

 Craspedochilus. 



Aplacophora. Worm-like Amphineura in which the foot has been 

 greatly reduced to a median ventral ridge and the mantle corre- 

 spondingly enlarged. Mantle cavity reduced to a small cloacal chamber 

 at the posterior end, gills present or absent, e.g. Neomenia. 



Craspedochilus is a small mollusc found underneath stones be- 

 tween tidemarks. It looks like an elongated limpet and has exactly 



