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 in 
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, and 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 the 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. CRANGON 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 antennule, has the form of a half-circle. In 
a cross-section more posterior its outline is made irregular by 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 Palz- 
monetes, where the sensory cushion is basal. More irregular still is its 
form in frontal section, as shown at ers. 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 
