32 PEOF. M. M. ILAETOG ON THE 



is that of a ganglionic enlargement. This is well seen in the living animal at the base 

 of the antennnle (animal prone), in the posterior end of the abdomen (where both of the 

 terminal branches of the abdominal cord swell into elongated ganglia of this kind 

 (animal lateral), or especially at the base of the fifth thoracic appendage (with the animal 

 supine) ; with care it may be detected also at the sides of the carapace (animal lateral). 



Sympathetic System. — The only trace of this I have been able to find is in the azygos 

 nerve running down from the inferior angle of the brain, which gives rise to a ganglion 

 in front of the mouth ; but its fibres seem to be exclusively distributed to the hypoderm 

 of the ventral face of the epistoma and labrum and the front wall of the mouth. 



The Nerve-terminations . 



Motor. — It is very difficult to trace these in most parts ; but one pair are most con- 

 spicuous, that of the nerves from the fourth ganglion to the great flexors, seen readily 

 in the living animal (supine). The eud is a typical Doyerian hill, containing several 

 small irregular refractive bodies, which are possibly nuclear. In one of my dissections 

 (gold chloride), I have found such a hill raised from its muscle (great flexor) iu an anterior 

 thoracic segment, and the base of the hill appears to run out at each end into a striated 

 muscular fibril. 



Sensory. — The ordinary ganglionic enlargement of the sensory nerves before its termi- 

 nation has been already described ; the fibres then run to the hypoderm, where they are 

 apparently lost. In some of my osmic-acid specimens I have detected, in surface view, 

 a fine reticulum in the hypoderm, recalling the finest corneal network ; but though this 

 may be nervous, I have been unable to satisfy myself that it is not a mere coagu- 

 lation-product. However, we usually find that everywhere in the neighbourhood of the 

 entrance of a nerve into the hypoderm the cuticle bears a numher of circumvallate setae. 

 A group of these lies in the forehead, between the corneal facets : these are the equivalents 

 of the well-developed " frontal organ " found in so many Entomostracans, as well as in 

 some Malacostracan larvae ; and this fact confirms their interpretation as sense-organs 

 everywhere. In the abdomen the symmetrical arrangement of the circumvallate setae 

 makes them conspicuous under high powers (living), a pair on the supraanal plate 

 being easiest to find. 



A second form of cutaneous end-organ is the hair proper. Every plume and hair of 

 the antennule receives a fibre which has passed through a bipolar ganglion-cell ; and 

 probably the same is the case with all the hairs, hooks, and spines of the body, though 

 not the teeth and ordinary fine fringing setae. The majority of the hairs and the 

 circumvallate setae seem endowed only with ordinary tactile sensibility, and that this sense 

 should require special extensions of the skin is only natural iu an actively swimming 

 animal frequenting the mazes of aquatic vegetation. 



Besides these, the above-described lancets and pale hairs found on the antennules, especi- 

 ally of the male, and on some of the oral appendages, appear to be olfactory or gustatory in 

 function ; for there is no real difference between these senses in an aquatic animal. Their 

 presence on the antennule is universally characteristic of the Crustacea, and their better 



