36 report — 1877. 



Report on the Present State of our Knowledge of the Crustacea. — 

 Part III.' On the Homologies of the Dermal Skeleton {continued) . By 

 C. Spence Bate, F.R.S. fyc. 



Correlation of Ajipendages. 



By the term " correlation " I mean a change of form associated with func- 

 tional variations, tho character of which is sufficiently distinct to produce, 

 both in appearance and application, an appendage that is essentially dif- 

 ferent from the type with which it is homologically connected. 



The eyes are less subject to correlate with other forms than most other 

 appendages. This probably arises from the circumstance that their func- 

 tional properties are only liable to vary in a greater or less degree of utility. 

 It is true that Alphonse Milne-Edwards has observed in a species of 

 Palinurus the eye to become altered into an antenna-like appendage : and 

 the author of this report contends that it is homologous with the first pair of 

 appendages in Nauplius, and therefore correlates with a free-swimming appen- 

 dage (Proceedings Hoy. Soc. vol. xxiv. p. 377) ; but our knowledge of the 

 cases is small where the eye loses its functional power ; consequently we must 

 assume that its variation in form must be limited in degree only consistent 

 with its uses. 



In Podopldhalma and allied genera the organ is extended on a very 

 long appendage, whereas in others the peduncle is extremely short ; and in 

 those genera that reside in dark habitats the visual organ has become so 

 depauperized that it can only be traced through the anatomical arrangement 

 of the nervous system. This is the case with the Cirripedia, where, from 

 the fixed nature of the animal, sight would only be a means of inflicting 

 pain, since the animal could not escape any object of terror it observed 

 approaching. 



The eye in the Nauplius form, whether in young specimens of the higher 

 types or adult forms of Entomostracous Crustacea, is not homologous with 

 the true orgaii of the higher forms of Crustacea, and therefore cannot be said 

 to correlate with it. 



The first pair of antennae, called the antcnnules by some writers, is 

 generally of a very simple character. They usually consist in their outward 

 form of a base or peduncle made up of three separate joints, the remaining 

 portion being broken up into numerous minute articuli, that gradually 

 decrease in size towards the extremity, and so become long and flexible, like 

 the lash of a whip, and consequently are named the flagellum by anatomists. 

 It is usual for this to break into two separate branches ; and it is clear that 

 one must be of a superior character to the other, since there are certain 

 organisms attached to it that are invariably constant, whereas they are never 

 attached to the other. I have therefore, when desirous to distinguish the 

 former, identified it as the primary branch of the flagellum and the other as 

 the secondary, which in different species is again liable to be rcdivided at 

 various points along the branch, but every time forming more feeble and 

 less important branches. This appendage, when compared with its homo- 

 type, a truly formed walking-limb, differs from it in the same way as the 

 latter changes in the lower forms of Crustacea when variatcd for other pur- 

 poses, as in Mysis. A true or normally developed limb adapted for walking 

 consists of seven separate joints. The first pair of antennas consists of seven 

 also ; but three of these only retain their normal character, the four others 

 being differentiated so as to comply with other conditions necessitated by 

 distinct wants. 



