THE DECAPODA 263 



which afford a remarkable instance of "convergence" in the assump- 

 tion of the Brachyuran facies, the relationship to the hermit-crabs 

 is shown by the fact that the short abdomen, which is closely bent 

 up under the cephalothorax, has the terga incompletely calcified, 

 and is, in the female, more or less unsymmetrical, bearing append- 

 ages only on one side. 



Among the Brachyura the abdomen is always closely flexed 

 under the cephalothorax, and is much reduced in size. The shape 

 usually differs much in the two sexes, being narrow in the male 

 but broad and often excavated for the reception of the eggs in the 

 female. The terga of all the six somites, as well as the telson, 

 may remain distinct, but very often two or three of the somites 

 may become coalesced, especially in the male sex. 



In the region of the thorax a system of internal skeletal structures 

 is developed by infoldings of the cuticle (apodemes) forming the 

 endophragmal system. In the Natantia, with feebly calcified integu- 

 ment, this system is but slightly developed, but in the Palinura and 

 Astacura, and especially in the Brachyura, it attains a great degree 

 of complexity. A " sternal canal " may be formed by the meeting 

 of the sternal apodemes of opposite sides above the nerve-cord, and 

 in the anterior part of the thorax this may give a firm plate or 

 " entosternite " lying between the nerve-cord and the alimentary 

 canal. It is not certain whether this entosternite involves any 

 elements other than those supplied by the ectodermal and cuticular 

 infoldings forming the apodemes ; if it does not it can hardly be 

 regarded as homologous with the entosternite already mentioned 

 in Branchiopoda (p. 44), which appears to be of mesodermal origin. 



In the Brachyura a sternal canal is not formed, the union of the 

 apodemes being confined to one or two of the posterior thoracic 

 somites, where it gives rise to a transverse bar known as the "sella 

 turcica." 



Appendages. Among the Decapoda the ocular peduncles (Fig. 

 154) assume more the character of limbs than they do in any 

 other Crustacea, since they are generally (perhaps always) divided 

 into two, or more rarely three, movable segments. Instances of 

 extreme development of the eye-stalks occur among Caridea and 

 Brachyura, sometimes the first (Podophthalmus, Fig. 154, C) and 

 sometimes the second segment (Macrophthalmus, Fig. 154, D) being 

 elongated. The corneal surface is generally terminal, but may be 

 oblique and even lateral, the peduncle running out beyond it into 

 a styliform process which may equal in length the rest of the eye- 

 stalk (Ocypoda, Fig. 155). In certain species of Gelasimus one of 

 the ocular peduncles terminates in a long process of this kind while 

 the other does not. In cases where the eyes are atrophied, as in 

 abyssal or cavernicolous decapods, the peduncle often persists in a 

 reduced state (Figs. 161, 162). 



