ON THE BRITISH EDRIOPHTHALMA. 25 



This law, which regulates the cliaracter of the cephalic segments in the 

 higher types, is still persistent in the Edriophthahna. 



The nervous system below the Stomapoda is entirely free from thoracic 

 consolidation, except in the abnormal class of Cirripedia. The cephalic 

 region or segments belonging to the organs of consciousness is reduced to a 

 minimum, or represented only by corresponding appendages. 



In all the higher types, the antennal segments as well as the mandibular, 

 excepting only the anomalous genus of Squilla and its near allies, unite to 

 build up the carapace, the respective relation of each segment to the others 

 differing in importance in distinct orders. This appears to be the same with 

 respect to the cephalic ring of the Amphipoda, which homologizes with the 

 entire carapace of the Brachyura and Macroura, differing from them only 

 in degree. 



In 4,he Macroura the development of the mandibular segment extends 

 back and covers the whole of the thoracic region, forming so efficient a pro- 

 tection as to render the completion of the dorsal portion of the thoracic seg- 

 ments a work of supererogation. These latter rings in the higher types 

 become so closely compacted together, that, by diminishing their extent, 

 they concentrate their force ; whereas in Amphipoda the thorax is developed 

 into seven distinct and perfect rings, while the homologue of the carapace 

 reaches not beyond the segment which bears the first maxilliped, and 

 this not by any extraordinary development of the posterior cephalic rings, 

 but by the consolidation of the three segments next succeeding the man- 

 dibular into one, which supports the three posterior appendages of the mouth. 



Prof. Milne-Edwards * contends that the whole of the seven anterior 

 segments of the animal are fused together and form the first or cepha- 

 lic ring. 



"The exact normal relation of the shell of the head," says Mr. Dana 

 (part i. p. 35 of his great work), " is with difficulty determined ; yet the argu- 

 ment that this segment extends across below just anterior to the mandibles, 

 and only here, probably holds in this group, as in the Decapoda, so as to 

 show that the shell pertains either to the mandibles or second antennae : 

 further investigation may possibly bring out a more definite decision." 



The effort in this Report will be directed, if possible, to demonstrate that 

 the "shell of the head" is homologically the same as the carapace in the 

 higher types, restricted according to a law of development to be a less im- 

 portant feature of the animal. Gradually it descends from the most per- 

 fect forms. 



In Macroura, a distinct suture, the cervical or epimeral of M. Milne- 

 Edwards, is visible, distinguishing the mandibular from the antennal seg- 

 ments. In Brachyura the large development of the antennal segments 

 completes most of the carapace; in Macroura the mandibular ring equals, if 

 not exceeds, the half of this structure. This change is produced in the rela- 

 tion of the two parts by a corresponding decrease of importance in the an- 

 tennal or cephalic portion, rather than by an extraordinary enlargement of 

 the mandibular. As we descend in the scale of Crustacea, we find that the 

 antennal, or that portion supplied with nerves from the cephalic ganglion, 

 diminishes in size in relation to the rest of the carapace, and that the cara- 

 pace likewise itself loses its importance in relation to the entire animal. 



This, which we see being carried out in the Macroura, Stomapoda, and 

 DiastyUdce, where the thorax of the animal is seen gradually in each suc- 

 ceeding form to become less protected by the carapace, appears to reach a 

 l^mit approaching the extreme in the Amphipoda, when the entire thorax is 

 * Histoire des Crustaces, vol. i. p. 20. 



