COMPARATR'E ANATOMY. 31 



sericea "The spicula remain in connection with the hypodermis by thin threads, 

 met with everywhere in tlie sjiicida" (cuticulaV) and of P. discoveryi he states 

 that "The spicules are formed in small accumulations in the epidermis" though 

 one cell only is said to be active. 



Thiele has consistently argued in favor of one matrix cell and it is probable 

 that he has focussed his attention on the type of spine I am about to describe. 



As noted in a preceding paragrajih, there are in addition to the tangentially 

 placed spicules in some species others that from the beginning to the close of 

 their development are directed radially. This latter type of spine, so far as I 

 have determineil, has a mode of development completely different from the one 

 just described. In Strophomenia triangularis for example as soon as it becomes 

 clearly recognizable it rests upon what appears to be a single cell, and as long 

 as it remains in the cuticle no additional elements put in an appearance. In the 

 later stages the matrix cell usually l)ecomes more cup-like closely clasping the 

 base or side of the spine (Plate 36, fig. 18), and it may elongate to form a slender 

 stalk but it is always unicellular. It is to be noted, however, that in this species, 

 and perhaps in others, there are additional, radially directed spines of much 

 larger size which apjjear to be formed bj^ more than one cell though this is not 

 certain owing to the fact that the base of each spicule is crowded against the 

 somatic musculature. 



In the Chaetodermatidae Wiren has shown that but one formative cell 

 exists though in its early stages the spine is surrounded by three hypodermal 

 cells which may exercise a moulding influence. From my studies I doubt 

 this last statement. In both Chaetoderma and Limifossor the formative cell is 

 surrounded by hypodermal elements but there is no evidence that in the develop- 

 ment of the spine they take any active part. It thus becomes evident that there 

 are various types of spicule formation among the Solenogastres just as there are 

 among the Chitons, but it is a most interesting and significant fact that the most 

 common type of Chiton-spine development (according to Plate, '01, Theil C, 

 p. 372) is almost the precise counterpart of what exists in P. hawaiiensis and 

 several other species. 



Wiren is inclined to the belief that after the spicule forming cells have 

 performed their function they become transformed into the hypodermal papillae. 

 There is confessedly no definite evidence to substantiate such a theory, and on 

 the other hand there are one or two facts that tend to discredit it. In the first 

 place there is no definite relation between the number of papillae and spines; 

 in the majority of species the latter considerably outnumber the former, notably 



