Common Crayfish. 91 



cephalo-thoracic carapace again is divisible into two regions, an 

 anterior and a posterior, by the well-marked curved line with its 

 concavity looking forwards, which is known as the ' cervical su- 

 ture/ The anterior portion of the carapace is called the ' cephalo- 

 stegite '/ and the posterior the ' omo-stegite -,' inasmuch as an 

 examination of the relation of the inner aspect of the cervical 

 suture to the hindmost of the three pairs of jaws, that is to say, 

 to the most posterior of the cephalic appendages, shews that the 

 anterior portion of the carapace corresponds with the head, and the 

 posterior with the post-cephalic segments. The crayfish furnishes 

 an excellent example of the general law that every segment carries 

 an appendage in the Crustacean, the first post-abdominal segment 

 in this (female) specimen being the only segment in which a 

 distinct pair of appendages is not readily recognizable. The pe- 

 dunculate position of the eyes in the Decapodous Crustaceans 

 makes it easy to understand, even without a reference to the 

 history of development, or to instances of abnormal replacement of 

 the eyes by articulated appendages % how the eyes may be regarded 

 as homologous with articulated appendages ; and that the eyes and 

 antennae do not really belong to the tergal aspect of the anterior 

 cephalic segment, may be shewn not merely from the history 

 of the reflection upwards and backwards of the developing cephalic 

 blastoderm, but also from the direction and relations of these 

 three pairs of sensory appendages in such Crustaceans as the 

 Squillina. With the history of the relations of these organs in the 

 developing Astacus, or with that of their disposition in the family 

 just mentioned, before the mind, it is easy to see how the oph- 

 thalmic peduncle may be spoken of as the most anterior of the ap- 

 pendages, and as marking out the most anterior of the segments 

 of the body. Next in order to the ophthalmic peduncles come the 

 ' antennules/ consisting, as usual in Crustacea, of three basal joints, 

 succeeded by a multi-articulate flagelliform element, which is here 

 double. The 'antennules' are said to correspond to the antennae 



" See A. Milne-Edwards, Comptes Rendus, vol. lix., p. 710, f., 1864. It may be 

 added that the pedunculate position of the two centrally-placed eyes in certain 

 Ephemeridae, for which see Westwood, Modern Classification of Insects, pp. 35, 31, 

 De Geer, Histoire des Insectes, viii., pi. 18, torn. 2, fig. 10; Kirby and Spence, pi. 

 xxvi, fig. 39, is, when we consider the many Crustacean affinities of the order Orthop- 

 tera, not without significance. 



