312 THE ANATOMY OF INVERTEBRATED ANIMALS. 



is produced anteriorly into a strong, curved, and pointed ros- 

 trum ; on each side it bears an aggregation of simple eyes, 

 and in front, immediately beneath the rostrum, this face gives 

 attachment to two long, many-jointed antennules. Below 

 these, two antennae, shorter, and fewer-jointed than the an- 

 tennules, are inserted, and the inferior part of the face is 

 completed by a large movable labrum. Behind this come 

 the strong, toothed palpigerous mandibles (IV), and two 

 pairs of more or less foliaceous maxillae. Inasmuch as the 

 eyes are sessile, these five pairs of appendages are all that 

 belong to the head proper ; but, just as in the Podophthal- 

 mia, certain of the anterior thoracic appendages are con- 

 verted into accessory gnathites, so, in Amphithoe, the first 

 pair of these members are applied against the mouth, and 

 form a large lower lip-(VII'). 



The " head" of Amphithoe, therefore, is formed by the 

 coalescence of the seven anterior somites of the body, but I 

 believe that the tergum of the seventh (or first thoracic) so- 

 mite is obsolete, as in a Stomatopod, and hence that the ter- 

 gal surface of the head of the PJdriophthahnia corresponds 

 exactly with the cephalostegite (or that part of the carapace 

 which lies in front of the cervical groove) in Podophthalmia. 

 Mr. Spence Bate has shown, in his valuable " Report on the 

 Edr (ophthalmia " that in the Crustacea at present under 

 discussion, a strong apodeme arises on each side from the 

 posterior part of the sternal region of the head, and passing 

 inward and forward meets with its fellow to form an endo- 

 phragmal arch, which supports the oesophagus and stomach, 

 and protects the nervous commissure between the first and 

 second sub-oesophageal ganglia, which runs under it. 



The discoverer of this structure conceives that it repre- 

 sents the terga of the three somites immediately succeeding 

 the mouth ; but I cannot see that it is other than the repre- 

 sentative of the precisety similar mesophragm formed by the 

 anterior apodemes in Astacus. In fact, the correspondence 

 in structure between the head of an Amphithoe and the ceph- 

 alic portion of the cephalo-thorax of Astacus is not a little 

 striking. There is the same sternal flexure, the same relative 

 position of the stomach, and of the insertions of the mandibu- 

 lar muscles. The great difference lies in the abortive condi- 

 tion of the ophthalmic appendages. 1 



1 A strong cndophragmal arch separates the snb-cesophageal ganglia and com- 

 missures from the gullet in Squilla, but has different connections (Fig. 83). _ A 

 very similar endophvagmal arch is found in the Insect head. See the descrip- 

 tion of the head of Blatta {infra). 



