30 COMPARATIVE ANATOMY. 



(see Plate 36). This latter feature becomes strongly marked beyond this stage 

 and as the cells shift to a distinctly subterminal position their boundaries dis- 

 appear, and in the neighborhood of the spine they become dense and fibrous. 

 Still later they migrate to a point fully one eighth the length of the spicule from 

 a terminal position, their attachment to the spicule sheath becomes reduced to a 

 minute kiiob-like disc and all but the basal portions of the cells themselves be- 

 come transformed into a slender fibrous stalk, which elongating as additions are 

 made to the cuticular investment of the body, maintains its attachment with 

 the spine as long as the latter remains in the cuticle. In the region of the hy- 

 podermis the outlines of the matrix cells remain distinct and unmodified with 

 the exception of the enclosed basal cell which becomes distinctly fibrous. In 

 certain slightly abnormal cases the stalk is sometimes unusually broad and 

 under such circumstances the basal cell in later stages becomes distinctly fibrillar 

 throughout its entire length, while the surrounding ensheathing cells assume 

 rather a cuticular appearance and never so far as I have seen assume a fibrous 

 character. In a very considerable number of cases the fibres, of unknown 

 nature, that have developed in the original basal cell may be seen to extend 

 beneath the level of the hypodermis, or to unite with others that may be traced 

 for varying distances into the somatic muscles beneath. In the region of the 

 buccal cavity, in Proneomenia hawaiiensis, they may be followed through spaces 

 in the muscle layer into close proximity to the ganglionic masses bordering on 

 the cirri. So long as the spicule remains imbedded in the cuticle the stalk is in 

 connection with it, and appearances suggest that after the basal cell ceases to be 

 functional as a spicule forming agent it may transmit impulses to the central 

 nervous system as the spines and scales in the mantle of the Chitons are sup- 

 posed to do. 



In Halomenia, Lophomenia, Dorymenia and all of the species of Stroph- 

 omenia described in the present paper this type of development prevails. In 

 some species the matrix cells become detached from the fully formed spine, 

 but otherwise no fundamental differences exist. Regarding the species described 

 by other authors, Hubrecht and Heuscher agree that in Proneomenia sluiteri a 

 cup of several cells clasps the base of the spicule, and Kowalevsky and Marion 

 and to a certain extent Pruvot, and Wiren have made similar observations on 

 other species. Through the generosity of Professor Hubrecht I have been able 

 to examine a portion of the type of P. sluiteri and though the cells are not so 

 clearly defined as in P. hawaiiensis there is no doubt that in both the spines 

 follow the same course of development. Nierstrasz writes of Cyclomenia holo- 



