108 Descri'ptions of Preparations. 



be recognized in Astacus as well as in other Arthropoda^ when the 

 specimen has been, as in this case, sufficiently hardened in alcohol. 

 The summits of the branchial plumes are well seen in this Prepa- 

 ration, between the omo-steg-ite and the epimera of the abdominal 

 segments. From the internal aspect of the epimera, the muscles 

 which move the limbs are seen passing downwards and bifurcating 

 as they pass through the apodematal cells to their insertions in the 

 upper segments of those appendages. In the cephalo-stegite, the 

 adductor mandibulae is seen taking origin from the internal aspect 

 of the protuberance already described, at page 96, as existing on 

 its outer svirface. 



On the ventral surface a packet of spermatozoa, aggregated in 

 their passage along the convolutions of the vasa deferentia into the 

 so-called ' spermatophore/ may be observed adhering to the cox- 

 opodite of the last abdominal segment, in which the vas deferens 

 opens. These structures, which are of comparatively small size 

 as compared with those of some much smaller Crustacea, but which 

 are produced in great numbers in the Crayfish, are by no means 

 found exclusively in the neighbourhood of the generative outlet 

 of the male, but may be seen adhering to various parts of the 

 animal's body. In the Chilopodous Myriapoda they may be at- 

 tached even to foreign bodies. 



It may here be noted that though in many Hedriophthalmata, or four- 

 teen-footed Crustacea, the number of post-oral ganglia may be the same, 

 or nearly the same, as in such Macrurous Decapods as the Crayfish or 

 Lobster, it is never made up of the same pre-thoracic, thoracic, abdominal, 

 and post-abdominal factors in the same proportions as in the higher 

 order. The ganglia supplying the ambulatory appendages, which in the 

 Hedriophthalmata correspond with the two posterior maxillipeds or 

 foot-jaws of Decapods, escape fusion with the first post-oral mass con- 

 sisting of the coalesced ganglia belonging to their three pairs of foot- 

 jaws and single pair of foot -jaws ; and maintain their typical distinct- 

 ness, just as the appendages which they supply maintain their primordial 

 locomotor functions. Having thus two more ganglia in the anterior 

 portion of their segmented bodies, the Amphipods and Isopods have 

 fewer than the Decapods in the post-abdominal region ; and by a re- 

 duction of the number of the ganglia of this region from six to five or 

 four, the entire number of their ventral gangfia comes to be thirteen 

 or twelve, as in Homarus or Astacus. In some of the air-breathing 



