THE THREE EYES OF MONSTRILLA. 25 



be produced. Hold a coin between finger and thumb, pull on the 

 uppermost point of the edge and the movement that produces 

 extension in crustaceans, is simulated ; if the lowermost edge be 

 pulled, then flexion is imitated. The anterior end of all these muscles 

 is attached well forward to the inside of the chitonous shell that 

 forms the protective skeleton' in these animals ; the hinder end being 

 inserted on the front edge of the body ring next following, and which 

 is governed in action by the muscle so placed. 



The swimming feet are each governed in their larger movements 

 by two great fan-shaped muscles which pass upwards from the thigh 

 joint along the sides of the body. The smaller movements are 

 co-ordinated by a very complex series of small muscles which space 

 forbids the enumeration of. The antennae are each governed by two 

 strong muscles passing backwards close to the eyes. In both sexes 

 the muscular development of the antennae is great, particularly in 

 the male, and is of great assistance to the animal in swimming, 

 being, in this sex, virtually equivalent to the addition of another pair 

 of swimming legs. 



Sense Organs. These seem practically limited to eyes and 

 sensory hairs. The former are well developed, three in number, each 

 possessing a large crystalline lens. They lie somewhat in triangular 

 fashion, two paired ones on the upper surface of the head between 

 the bases of the antennae, and a third, the unpaired one, being 

 beneath these on the under side of the head. The structure is 

 essentially that of a convex glassy lens set in a mass of pigment and 

 connected by a stout nerve with the brain. 



It is interesting to notice that the paired eyes look sideways, 

 while the unpaired one looks downwards and forwards, just the 

 best possible optical arrangement for the animal when it betakes 

 itself to free life at the surface of the sea. To pelagic animals there 

 is no above. Their world lies around and beneath, and there alone do 

 they require to cast inquiring glances, if indeed their modest optical 

 equipment will so permit me to phrase it. 



In the females full of ova, the eyes are frequently much reduced 

 and apparently more or less absorbed and functionless. 



As to sensory hairs, there are apparently two kinds, one having 

 a much branched form (Fig. 10, a), the other straight and un- 

 branched (6). It may be that a is olfactory and b tactile, but this 

 rests entirely upon supposition. 



Respiration. No organs adapted to this end. The general 

 surface of the body seems to function. 



Reproduction. The sexes are always separate ; the difference 

 expressing itself plainly in the outward form. The male is smaller 

 (2'25 mm.) than the female (3 mm.) ; he has more powerful antennae 



