GENERAL CONSIDERATIONS. 251 



GENERAL CONSIDERATIONS. 



Since the completion of the report on certain species of Solenogastres from 

 the Pacific Ocean (Heath, 1911) the excellent paper of Nierstrasz (1908) and 

 several objections {in litt.) on the part of various investigators open up the dis- 

 cussion of certain questions not fully treated in the earlier paper. It is evident, 

 however, that some of these protests, like many of my own theoretical con- 

 clusions, are very largely based on personal opinion. The same material in 

 the hands of these or other students would perhaps be interpreted in various 

 ways from a theoretical standpoint, and accordingly the following paragraphs 

 are very largely a confession of faith with some of the grounds upon which it 

 rests. 



The first of these criticisms is directed at the section treating of the forma- 

 tion of the spicules imbedded in the cuticle, which, hke the papillae when such 

 are present, is a product of the hypodermis or epidermis as Nierstrasz prefers 

 to term it. At the outset it is important to note that there are two distinct 

 modes of spicule formation, and the confusion that my account appears to have 

 created is largely due to the fact that this has not been kept in mind. In all 

 of the Chaetodermatina, so far as my observations go, each spine is the product 

 of one, and only one, cell, and it may be, indeed it usually is, crowded between 

 adjacent hypodermal (or epidermal) cells. But that these surrounding cells 

 are limited to three, or that they perform a molding function as Wiren main- 

 tains (Wiren, '92) is open to serious question. In those sporadic cases where 

 the matrix cell is raised above the general level of the remaining elements of 

 the hypodermal layer the minute spine is clearly seen to rest solely upon this 

 formative cell, and is not in intimate contact with any other cell element. The 

 same method of growth also appears to be characteristic of the families Neo- 

 meniidae and Dondersiidae. In those species of the suborder Noemeniina 

 where the spicules form more than one layer a relatively small number of spines 

 are usually directed radially and at the completion of their development project 

 beyond the external surface of the cuticle. So far as I have been able to follow 

 the development of all such radial spicules each is the product of a single cell, 

 which is either attached directly to the base or close to the base at one side. 



In the families Proneomeniidae and Pruvotiniidae the development of 

 the tangentially placed spines follows a different course of development, at 

 least in several carefully studied species belonging to the genera Halomenia, 



