OPTICAL INSTRUMENTS 35 



tion. It is unusual to think of a creature's eye as one of 

 its limbs, for only by a figure of speecli do we describe a 

 person as grasping the whole situation at a glance. Never- 

 theless, there are very few inclined to dispute that the 

 eyes of the Podophthalma have really been developed upon 

 appendages by nature equivalent to the rest of the series. 

 Any one acquainted only with the extremely short-stalked 

 eyes of some of the crabs might be excused for thinking 

 this view extravagant, but its improbability is lessened 

 when we observe the long-stalked eyes of the Angular 

 Crab, Gonoplax rhomboides (Linn.), or those of the macruran 

 Leuoifer Reynaudii, Milne-Edwards, or those, again, of 

 Eretmocdris longicaulis (see T?\ate XIII.), a shrimp in.'which 

 the eye is projected on a support actually longer than the 

 animal. Moreover, the ocular appendage, besides being 

 articulated to thehead, is itself composed of two or three 

 articulations. In the fast-running Ocypode cursor (Linn.), 

 the peduncle is extended beyond the cornea of the eye, 

 and terminates like an antenna in a pencil of long hairs. 

 There is one instance on record in which the eye of a kind 

 of lobster, PanuKrus penicillatus (Olivier), has been observed 

 to develop a jointed antenna-like lash, while the com- 

 panion eye remained normal. This evidence is parallel 

 to that on which a botanist infers that the petals of a 

 flower are by origin modified leaves when he sees them 

 occasionally assuming the form of the unmodified leaf. 



2. The second segment carries the first pair of an- 

 tennae, sometimes called the inner or upper, or, without 

 epithet, the antennules. In the Malacostraca normally 

 these appendages consist each of a three-jointed peduncle 

 and two flagella or lashes, composed of many joints or 

 few, the inner or secondary flagellum being not unfre- 

 quently absent altogether or rudimentary. In some in- 

 stances, and in the Amphipoda Oaprellidea and in the 

 Entomostracan Copepoda not as an exception but as the 

 rule, the first antennae are larger than the second, from 

 which it results that the diminutive name antennules is 

 -rather convenient than appropriate. The superior size, 

 however, is no indication of higher rank, but rather the 



