166 GENERAL CONSIDERATIONS. 



are in contact with it, and the attachment may persist for a considerable period. 

 In Proneomenia hawaiiensis and a number of other species I have found that 

 there is one basal cell, apparently responsible for the formation of calcareous 

 material, surrounded by seven or eight smaller cells attached also to the base 

 of the spine and perhaps responsible for the formation of the cuticular sheath. 

 This last named mode of formation is almost the exact duplicate of the most 

 common method of spine development of the Chitons (Plate, 1901, p. 372). 



Among the Chitons the matrix cells retain their connection with the spicule 

 as long as it exists. In the Chaetodermatidae, and in those Neomeniina with a 

 single layer of spines, there is a tendency to follow this primitive method. The 

 same is true, though in some cases to a more limited extent, in a few species 

 with thick cuticle and several layers of spicules, notably P. hawaiiensis and 

 Slrophomenia scandens. 



The balloon-shaped papillae developed from the hypodcrmis and in a fully 

 developed condition extending to the free surface of the cuticle are of proble- 

 matical significance. It has been suggested (Kowalevsky & Marion, Wiren) that 

 they may be spicule-matrix cells that have assumed some new function after 

 the formation of the spine; but the fact that in P. hawaiiensis the matrix cells 

 retain their attachment, at least in part, as long as the spine is imbedded in 

 the cuticle, precludes such a possibility. In Alexandromenia there are many 

 times more spines than papillae. Heuscher on the other hand describes their 

 origin as simple outgrowths of the hypodermis. Regarding their homologue in 

 the Chitons nothing may be claimed definitely. They may correspond to the 

 packets or papillae (Plate) or, with a greater degree of probability, to the 

 aesthetes as several authors have claimed. 



That a foot of much larger size existed in the ancestral Solenogastre and 

 that the ventral groove of the Neomeniidae does not "represent the first stage 

 in the formation of that pedal surface of the body which is seen in the lowest 

 mollusca" (Gegenbaur) is indicated by a number of facts. In the first place 

 although no external trace of a foot exists in the Chaetodermatidae there is a 

 space along the mid ventral line between the longitudinal somatic muscles which 

 are thicker here than elsewhere, and in Limifossor this same space is occupied 

 by a sinus exactly similar to the pedal sinus of the Neomeniidae. In Limifossor 

 the pedal sinus anteriorly penetrates a clearly defined septum and communicates 

 with the head cavity as in the Chitons. It is much more reasonable to consider 

 that the pedal sinus is the remnant of the foot of the ancestral Solenogastre 

 than that it is the first sign of the appearance of a definite creeping surface. 



