28 COMPAUATn'E ANATOMY. 



fig. 5) and come in contact with tlic under sui'faco of modified papillae, which 

 appear to be capable of a cei'tain amount of protrusion t)wing to the agency 

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

 is very obscure; and for tliat matter the various fimctions such as excretion, 

 touch, and pressure relations which ha\e been asci'ilied to these organs as yet 

 rest upon no direct experimental evidence. That they ari' the liomologue of 

 the aesthetes in the Chiton shell is a reasona])le 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 variety 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 may be associated radially directed spicules usually with truncated 

 ba,ses. 



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

 notably Pronrumvnid lutwuilcnsh, Slrophonicnia scandens, and Hahtmcnia (jravidii, 

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

 of the body appear with unusual distinctness, and to avoid needk^ss repetition 

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

 of Proneomenia hawaiicnsis, 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 \n' taking part. One of somewhat larger size than the 

 others, and with clear finely vacuolated cytoplasni and distinct granular sphei'ical 

 nucleus, rests underneath the base of the s]iine. Its g(>ncral appearance is 

 essentially like that of the cell beneath the spicules of (,'. in'tiilnluin as figured by 

 Wiren or the s]Mcule fornung cells in the mantle of c(>rtain sjiecies 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 hyixidernfis. In all of the Soleno- 

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

 distinct cell membi'ane anil cl(\irly different from any of the hyiiotlci'nial cells. 

 Furthermore in the species undi'r discus-idu I have never seen these jilasma cells 

 outside of the somatic muscle layer, anil there are never any indications that the 

 spicule fornung elements are tlerivetl from any other soui'ce than the hypodermis. 



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 are imbedded. This spicule 



