252 GENERAL CONSIDERATIONS. 



Dorymenia, Lophomenia, and Strophomenia. At its first appearance the 

 minute, cone-shaped calcareous product rests upon a relatively large cell that 

 probably may be considered the homologue of the matrix cell in the foregoing 

 types. In this case, however, the matrix cell is surrounded by "seven or eight 

 cells, slender in form, indistinct in outline, with dense nuclei and attenuated 

 bases which are imbedded in the hypodermis proper." The cell membranes 

 of these accessory elements are distally attached to a membrane or sheath 

 enveloping the spine which accordingly is interrupted near the base. The 

 part played by these different cells is obscure. The basal cell doubtless acts 

 as a Ume-secreting agent, and the remaining subsidiary cells form the spicule 

 sheath, but whether they hkewise supply calcareous material remains unde- 

 termined. The important point, however, is clear that all of these cells, eight 

 or nine in number, are attached to the spine, diminish in size as the spine 

 increases, and in many instances retain their attachment permanently. There 

 is therefore nothing to indicate that they are other than matrix cells. Essen- 

 tially the same mode of development has been described by Plate in his study 

 of the formation of the spines of certain species of Chitons, and it furnishes 

 another hne of evidence for the beUef that the Solenogastres and Chitons have 

 had a common ancestor. 



One point noted in the Pacific report remains obscure. In early stages a 

 minute body rests between the basal matrix cell and the base of the spine. 

 The point in question is whether it is a cuticular product or a cell. In a late 

 stage it undoubtedly is cuticular, and appears to prevent the passage of lime 

 salts from the matrix cell and by its increase and subsequent decrease and final 

 disappearance is responsible for the development of the cavity within the spine. 



At the suggestion of Professor Nierstrasz I have made a careful examina- 

 tion of the heart in all of the species of Solenogastres described in the Pacific 

 and in the present report ; as a result I cannot feel that much dependence can be 

 placed on this organ as indicating relationships or relative primitiveness consid- 

 ering our scanty knowledge of the group. To me it appears clear that the dorsal 

 blood vessel in the pericardial region has been provided with a highly developed 

 muscular coat, has thus become a pulsatile organ which frequently comprises 

 two divisions, a ventricle and auricle or atrium, as certain authors prefer to 

 term it. In some species this muscular section lies in a dorsal fold of the peri- 

 cardial wall; in other cases it has severed its connection with the wall and Ues 

 freely in the pericardium. In certain species the auricular or atrial division is 

 very short, as in Chaetoderma argenteum, or it may be more pronounced as in 



