196 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



sensory ridge. The origin of these could not with certainty be traced 

 out in the material at command, though from the conditions found iu 

 the first stage, it is probable that they are derived from the neuroblast 

 cells of the brain. The only evidence in favor of this view is the prox- 

 imity of the brain, aud the fact that at an early stage nerve cells which 

 were continuous with the ganglionic masses of the brain were present 

 beneath the matrix cells of the otocyst. Figure 26 shows, somewhat 

 diagrammatically, the general innervation of the otocyst hairs of the 

 fourth larval stage, as brought out by methylen blue. The condition is 

 essentially that of the adult. There is but one nerve element to each 

 hair, and the endings are -in the enlarged bases. No myelin sheath is 

 developed in either tlie larva or adult lobster. Central terminations of 

 the otocyst fibres were not traced out, nor was their finer histology 

 investigated. 



The most striking point to be noted in the development of the otocyst 

 of the lobster is the abrupt change which takes place after the third 

 moult. The shallow, functionless depression of the third stage is con- 

 verted at once into the active, well-differentiated organ of the fourth 

 larva. This sudden leap in the development of the otocyst is correlated 

 with an abrupt metamorphosis of the larva's general form and method 

 of locomotion. As this correlation may have an important physiological 

 significance, it will be discussed in detail in the theoretical portion of 

 this paper. 



II. Crangox vulgaris Say. 



1. Structure of the Otocyst. 



a. Sac. The otocyst has been described only briefly by Hensen ('63). 

 He figures the sac dissected out, and gives two sketches of the sensory 

 hairs, and the prominence upon which they are borne. 



The sac, as seen in a section passing through its middle and trans- 

 verse to the long axis of the anteniiule, has the form of a half-circle. In 

 a cross-section more posterior its outline is made irregular b^' the pro- 

 jection of the sensory ridge or cushion from its lateral wall (Plate 6, Fig. 

 28). This is an entirely different condition from that found in Pahe- 

 monetes, where the sensory cushion is basal. !More irregular still is its 

 form in frontal section, as shown at crs. sns. in Figure 29 (Plate 6). The 

 dimensions of the sac in individuals of medium size (25 mm. long) are : 

 length 0.44 to 0.55 mm. 



width 0.28 " 0.38 " (anterior to sensory ridge) 

 depth 0.20 « 0.22 " 



