310 MOLLUSCS [CH. 



genital organ is a median mass of cells attached to the front wall of 

 the pericardium and bursts into the pericardium when it is ripe. 



The Solenogastres have no foot, but in place thereof there is in 

 most of the genera belonging to this group a median ventral groove 

 clothed with thick cilia in which there is a very slight median ciliated 

 elevation which has been regarded as the first rudiment of a foot. 

 It is to be surmised that the animals use these cilia to glide over 

 the mud in which they live. This groove though absent in the 

 genus Chaetoderma has given the name to the group, for Soleno- 

 gastres means " grooved -bellies " (Gr. crwXrjv, a groove). It is one of 

 the most interesting discoveries of the science of Embryology that 

 the more primitive Mollusca and Annelida have in their life history 

 a type of larva called the Trochophore which is almost identical in the 

 two groups and which is regarded by many zoologists as an indica- 

 tion that both groups are descended from a common ancestral form 

 which in some respects resembled the Trochophore. Now in many 

 forms of Trochophore larva there is a ventral ciliated groove and it 

 may be surmised that the Solenogastres diverged from the common 

 ancestral stock of Mollusca very soon after this stock had become 

 distinguishable from the Annelida. The genus Neomenia has been 

 dredged in the Firth of Clyde. 



Class IV. SCAPHOPODA. 



The members of this group comprise a few species referable to 

 two or three closely allied genera. The name, which means "spade- 

 footed " (Gr. o-Ka'^og, a spade), is unfortunate because although the 

 foot is used for burrowing into the mud in which the animals live 

 there it nothing distinctive about this, for most Pelecypoda (Lamelli- 

 branchiata) also possess a foot which is used for burrowing into 

 mud. The foot of Scaphopoda is a cylindrical organ which has a 

 trilobed termination. The lateral lobes of this are capable of being 

 spread out horizontally and so give the foot, when it is driven in, a 

 firm hold on the mud. 



The distinctive characteristics of the Scaphopoda are to be found 

 in the mantle and the shell. The latter is tubular and curved, 

 shaped something like an elephant's tusk, it is open at both ends, 

 but the larger opening is the one through which foot and tentacles 

 are protruded and also the one through which faeces are ejected as 

 well as that through which food is taken in, so that the upper and 

 smaller opening is comparatively functionless. The mantle is also . 

 tubular like the shell which it secretes ; but in the young Scapho- 



