xlviii THE VOYAGE OF H.M.S. CHALLENGER. 



In some genera of the Schizopoda, which are aberrant Macrura, the inner branch is 

 furnished with an otolith, similar to that which we find in the first joint of the peduncle 

 of the first pair of antennas in some of the Phyllobranchiata. The analogy that it bears 

 to the first pair is moreover apparent in the filamentary character it assumes in Tanais 

 and Apseudes, in the latter of which it exhibits the unusual feature of two filamcntose 

 branches, which is the normal condition of the Macrural antennae. 



Tlie Telson. — The terminal or twenty-first somite — the seventh somite of the pleon — 

 undergoes considerable degrees of modification throughout the several families of the 

 Macrura. 



In most genera the telson appears to be useful as being the resting-place of the 

 sphincter muscles that surround the terminal extremity of the alimentary canal, which 

 is capable of being controlled by them. It is also useful in directing and steering the 

 animal in its passage through the water, and appears to be longest in those species that 

 possess the greatest power of rapid movement. 



All these animals possess the power of doubling up the posterior somites against the 

 ventral surface of the pleon, and then, by boldly striking out, dart to a considerable 

 distance. 



In some genera, or even families, the telson is posteriorly rounded, as in the Astacidse; 

 in others it is anteriorly hard and calcareous and posteriorly soft and membranous, as in 

 the Synaxidea, a circumstance that is suggestive of a distinct relationship of the two 

 parts, the anterior which carries the anus belonging to the normal somite, while the posterior 

 portion represents its appendages. This idea is still more strongly suggested in the genus ' 

 Cheiroplatea, where the separation of the posterior from the anterior division is clearly 

 defined by a distinct membranous articulation, and the posterior portion is divided into 

 two lateral lobes. 



In Glyphocrangon the telson is not only a long and slender appendage but it is one 

 that from its character and power must be a formidable weapon of offence. It is 

 developed in the form of a long, slightly curved, triangular bayonet, grooved along the 

 upper surface, and capable of being firmly fixed or unlocked at will. 



The contraction of the extensor muscle forces the ball-like portion at its anterior 

 dorsal margin beneath the frontal surface of the preceding somite, and draws the dorsal 

 process at the base of the telson into contact with the vertical margins at the posterior 

 extremity of the sixth somite, and by the same action the lateral bolts are forced against 

 the curved margin of the projecting lateral process on each side ; by these means the 

 telson is so securely locked in position that it is difficult to dislodge it when so fixed 

 (Fig. XV.). To add to its power as a weapon of offence the sixth somite is attached to 

 the fifth by a similarly formed articulation, which is also easily capable of being locked in 

 position, and the fifth somite is united with the fourth by a modification of the same 



