THE CRUSTACEA 



In Cijclops it has l^een observed that the pair of nerves to the 

 antennae originate from the oesophageal connectives, so that 

 as regards their nerve-supply these appendages are parastomial. 



Sense-Organs. — The paired compound eyes of other Crustacea 

 appear to be unrepresented in the Eucopepoda, although rudiments 

 of them were observed by Grobben in the development of Cuhmus. 

 The naU'plius-ei/e, on the other hand, is almost uni^'ersally present 

 in the free-living Eucopepoda, and even in many of the parasitic 

 forms, and in some cases, especially among the Clymnoplea, it 

 attains a complexity of structure not observed in any other class 

 of Crustacea. 



In the simplest and typical form it consists of three ocelli, each 

 supplied by a separate nerve from the brain. Tvro of the ocelli are 

 dorsal and look upwards and forwards, while the third is ventral, 

 looking downwards. Each consists of a cup-shaped mass of pigment, 

 containing in its cavity a number (up to ten) of retinal cells con- 

 tinuous at their distal ends with the nerve-fibres, and having (at 

 least in some cases) a rhabdome near the proximal end. In the 

 genus Anomalocera (Pontellidae) and in some Asterocheridae the 

 number of ocelli is increased to five, those of the dorsal pair being 

 doubled. 



In some Eucopepoda the eye is movable by means of special 

 muscles. These are wanting, however, in many cases {Cyclops). 



In some cases the visual apparatus is perfected by the addition 

 of a pair of corneal lenses formed by thickening of the cuticle 

 over the dorsal pair of ocelli. These may be inconspicuous as in 

 Cyclops, or large and well-defined as in Miracia (Harpacticidae). It 

 is, however, in the two widely separate families of Pontellidae and 

 Corycaeidae that the structure of the visual apparatus reaches 

 its highest degree of complication. In the former the dorsal ocelli 

 are often provided with cuticular lenses, and in addition there may 

 be developed a vesicular crystalline body interposed between the 

 retinal cells and the cuticle. The ventral ocellus approaches the 

 sternal surface and sometimes projects as a papilliform or peduncu- 

 late prominence, while the deflected rostrum in front of it becomes 

 thickened in such a manner as to form a biconvex lens, serving to 

 concentrate the rays of light upon it. 



In the Corycaeidae (Fig. 47) the three ocelli are widely 

 separate and the median element remains small, -while the dorso- 

 lateral pair attain a much greater — sometimes relatively enormous 

 — development. Each is provided with a large biconvex cuticular 

 lens (/), and the retinal apparatus is at a considerable distance 

 from this, at the apex of a conical space the base of which is 

 formed by the lens and the walls by a delicate membrane. The 

 pigment-cup {p) is elongated into a tubular form and at its mouth 

 is set a vesicular " crystalline body " {c). In Corycaens the posterior 



