340 Introduction to Animal Morphology. 



nerve. Closed vesicles are found in the larva of the crab, 

 containing an otolith, which is afterwards lost. They are 

 also closed in Hippa, Pinnotheres, Hyas, Ocypoda, Gelasi- 

 mus, &c. There is no ear in Crangon, Pandalus, Phyllosoma, 

 Erichthus, Thysanopoda. In Mysis it is a closed vesicle, 

 with an otolith attached by two hairs, and it is placed at the 

 base of the inner appendage of the tail-fan, receiving its 

 nerve from the last ganglion. 



The eye is only absent in cave-dwellers like 

 Astacus pellucidus of the Kentucky cave, and may 

 be of several types. The essential part consists of a 

 nerve fibre, ending in a long- crystal cone, whose apex 

 is directed inwards, merging into the nerve, and 

 whose base is highly refracting and turned outwards. 

 There is no special lens, but the chitinous integument 

 is transparent where it covers the organ, which in the 

 simplest case is quite free, and capable of sub-dermal 

 motion.* Such single crystal cones exist in Cope- 

 pod larva?, one on each side of the brain ; sometimes 

 a cluster of these may be present (Corycaeidae, &c.), 

 either medial, Cyclopean, or bilobed. Sometimes two 

 pair of these clusters are present, the outer of which 

 may develop large crystal cones, while the inner 

 fuse medially, and are rudimental, giving the appear- 

 ance of three eyes. 



In higher forms the chitinous integument becomes 

 moulded on the front of the crystal cone as a corneal- 

 lens, and is thus promoted into being a part of the 

 organ. Usually several, or many such crystal cones, 

 imbedded in pigment, are united, forming a compound 

 eye. The corneal lens may be simple over the cluster 



* In Argulus the eyes lie in a blood sinus in the head-shield. In many 

 Copepoda a muscle passes from the wall of the orbital cavity to the eye- 

 ball to move it. 



