1388 



CRUSTACEA. 



racter of the legs of insects, and the usual structure of the antennae in 

 Crustacea, we may justly infer that this abnormal number arises in 

 part, at least, from a subdivision of the terminal part of the legs. In 

 a figure which Kroyer gives of a young of Nymphon grossipes* the 

 first pair of these legs has the full number nine; the first three of 

 these short, the next three long, the seventh short, eighth long, the 

 ninth a claw; the second pair has but seven joints, the first three 

 short and the next three long, just like those of the first pair; the leg 

 in this condition has seven joints, and looks like ordinary Crustacea 

 legs, with the terminal claw. It hence follows that the additional 

 joints are formed at or near the extremity, and probably by a sub- 

 division of the penult. 



The third pair of legs in the same figure are represented as partly 

 developed, being only four-jointed ; the first joint is here the longest, 

 and evidently includes the first three of the perfect leg, and these 

 result by natural fission. The second pair of legs is therefore normal 

 in its joints, except that it has one too many; but this is probably the 

 first, for in many Amphipoda and Isopoda the epimeral segment is 

 properly a basal joint to the legs, and if counted, would make the 

 number seven. The first joint in the legs may therefore correspond to 

 the epimeral segment in other Crustacea. 



This subject derives much interest and some elucidation from a 

 comparison with the structure in the Arachnida, and also reflects 

 light upon that and other departments of Artie ul at a. 



In the Arachnida, the mouth organs consist of — 



1. A pair of mandibles. 



2. A pair of maxillae. 



3. A lower lip corresponding to a second pair of maxillaB. There is 

 also a simple upper lip which comes into the series. 



Judging both from embryology and the simplest forms of articulate 

 life, the Rotatoria, we may believe the mandibles to be the fixed deter- 

 minate centre through the whole series. The simplest form of life 

 above vegetation, has a mouth opening, and the first step above this, 

 consists in the presence of a pair of mandibles ; and from this pair as 

 the centre, developments go on towards the more complex forms. 

 This fact gives the highest importance to these organs as marking a 



* Toy. Scand., pi. 39, f. 1 d. 



