210 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



We are thus led to regard the hook hairs of the crab as homologues 

 of the otolith hairs of Macrura, and for these five reasons : — (1) The 

 similarity in their structure. (2) Their similarity in position at the 

 posterior end of the sac. (3) Otoliths are in contact with the hook 

 hairs in larval stages^ though not in the adult. (4) When the otoliths 

 disappear, the development of the hook hairs is arrested. (5) Gland 

 pores open through the chitin of their cushion, as they do through that 

 of the crayfish and lobster, although they are not found in the other 

 sensory regions of the sac. 



(2) The thread hairs are the largest, the most highly differentiated, 

 and probably the most active sensory bristles of the otocyst. There 

 are about thirty of them, arranged upon the large anterior sensory 

 cushion in a regular row (Fig. A, set. Jil.). These hairs are extremely 

 attenuate. Measuring only two or three fx at the base, the straight or 

 slightly bending shaft averages 320 ^ in length ; it is unfringed save 

 at the very tip, where for a short distance it bears two rows of ex- 

 tremely delicate pinnules. A peculiarity of this friuged tip is that 

 it is not a continuation of the main shaft of the hair, but seemingly 

 a diminutive hair in itself, sprouting from the latter. It makes a 

 slight angle with the main shaft, the end of which pi-ojects a short 

 distance beyond the base of the offshoot (Plate 10, Figs. 53, 54). 

 The shafts of these hairs are directed out laterally, and slightly pos- 

 teriorly, into the fluid contents of the sac, and they are so delicately 

 attached at their bases that the slightest jar imparted to the liquid 

 in which they float is suflScient to set them swaying. In alcoholio 

 matei'ial they break off very easily. The shaft decreases somewhat in 

 diameter towards its base and then suddenly enlarges. This enlarge- 

 ment is attached to the floor of a deep cup-like socket, the orifice of 

 which is large enough to give ample play to the shaft in its movements 

 (Fig. 53). 



Straight attenuate hairs are found in the otocyst of the Megalops 

 larva having the same relative position in the sac as the thread hairs 

 of the adult. These hairs are not in contact with otoliths, but each 

 shaft is fringed with filaments throughout its whole length. They 

 become differentiated in later stages into the peculiarly modified thread 

 hairs. Hairs similar to those of the Megalops larva just described 

 are also found in the otocyst of the adult lobster, situated on the 

 median wall of the sac and projecting free into its lumen. They are 

 similar in both larva and adult, and are probably in function accessory 

 to the otolith hairs. They may be homologues of the thread hairs, 



