28 COMPARATIVE ANATOMY. 



fig. 5) and come in contact with the under surface of modified papillae, which 

 appear to be capable of a certain amount of protrusion owing to the agency 

 of a surrounding blood sinus. The significance of tliis remarkable state of affairs 

 is very obscure; and for that matter the various functions such as excretion, 

 touch, and pressure relations which have been ascril)ed to these organs as yet 

 rest upon no direct experimental evidence. That they are the homologue of 

 the aesthetes in the Chiton shell is a reasonable assumption, but this carries 

 with it no trustworthy evidence regarding their office. 



The spines form from one to several layers in the cuticle, and present a 

 great varietj- of forms. In the Chaetodermatina spearhead-types prevail, 

 and in the Neomeniina, where there is but one layer, this shape may likewise 

 occur. In those species with more than one layer the usual type is needle-.shape, 

 and with it maj' be associated radially directed spicules usually with truncated 

 bases. 



Spicule Development. — In a number of species of the present collection, 

 notably Proneomenia liaivaiiensis, Strophomenia scandcns, and Halomenia gravida, 

 certain of the more important details of the formation of the spiculose investment 

 of the body appear with unusual distinctness, and to avoid needless repetition 

 the results are discussed once for all in the following paragraphs. Speaking first 

 of Proneomenia liaivaiiensis, in the earliest stages of the spicule formation, where 

 the calcareous product is no larger than the neighboring hypodermal elements, 

 several cells are seen to be taking part. One of somewhat larger size than the 

 others, and with clear finely vacuolated cytoplasm and distinct granular spherical 

 nucleus, rests underneath the base of the spine. Its general appearance is 

 essentially like that of the cell beneath the spicules of C. nitiduluin as figured by 

 Wiren or the spicule forming cells in the mantle of certain species of Chitons, 

 and is par excellence the lime secreting element. 



Wiren is of the opinion that the basal cell is a modified wandering cell that 

 has left the blood stream and migrated to the hypodermis. In all of the Soleno- 

 gastres under discussion the wandering cells are of a granular character with no 

 distinct cell membrane and clearly different from any of the hypodermal cells. 

 Furthermore in the species under discussion I have never seen these plasma cells 

 outside of the somatic muscle layer, and there are never any indicat ions that the 

 spicule forming elements are derived from any other source than the h>-podermis. 



In very early stages, perhaps from the first, the spicule is surrounded by a 

 delicate cuticular sheath whose reaction to the ordinary stains indicates a com- 

 position unlike the material in which the spicules arc imbedded. This spicule 



