GENERAL CONSIDERATIONS. 105 



increasingly evident that they possess more characters in common with the 

 Chitons than with the other classes, and these characters, interpreted in the 

 light of community of descent of the two groups, are more readily understootl 

 than from any other viewpoint. 



It is little more than waste effort at the ]iresent time to attempt to recon- 

 struct the external characters of the ancestral Solenogastre, for it is generally 

 agreed that the present thiy species, worm-like in form and without shell oi' 

 well-developed foot, are highly modified in these resjiects. The aggravatingly 

 few facts of their embryology are also without nuich value for the .solution of 

 the problem. As the matter now stands there is no positive evidence that they 

 ever had a shell, but in view of the fact that these animals show a close resem- 

 blance to the Chitons in several other respects it is not unreasonable to believe 

 that one was formerly present. It must be admitted that Pruvot's figure and 

 description regarding the shell in the larvae of Dondersia banyulensis are very 

 indefinite, and have led some authors to claim that the seven valves of the dorsal 

 side are in realitj' greatly enlarged scales. There are some evidences that such 

 is indeed the case in the young of Hohnncnia gravida, at all events the plates do 

 not develop exactly as does the Chiton shell. It is possible that we have here 

 the confirmation of Blumrich's theory that the original shell arose by the ex- 

 cessive development of flattened spines along the dorsal surface of the animal. 



It is a significant fact that the mantle of the Solenogastres has no counter- 

 part save in the Chitons. In the least modified condition it consists primarily 

 of a single layer of epithelial cells overlaid by a cuticular covering often of enor- 

 mous thickness. Those probably responsible for the formation of the cuticle, 

 and of pigment when such is present, are comparatively simple, unmodified, 

 more or less columnar cells. At frequent intervals throughout the layer spicule- 

 matrix cells arise and develop a single stratum of spines, or several layers im- 

 bedded in the cuticle. In their mode of origin the spines of the Solenogastres 

 are essentially similar to those of the Chitons (see p. 29). Thiele declares that 

 the Solenogastre spicule is produced from one matrix cell, but this method is 

 certainly not frequent, and Plate states that it is rare among the Chitons. Wiren 

 reports that in Chaetoderma there is one basal cell and three smaller ones en- 

 compassing the young spine. Hubrecht discovered that in Proneomeiiia sbiiteri 

 the base of each spicule is grasped by a considerable number of matrix cells, 

 and as the spine is carried outward by the continued growth of the cuticle they 

 continue to retain their attachment for a long period. Pruvot ('90) finds in a 

 few species that during the early development of the spicule four or five cells 



