ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 79 



In Astacus, where the structure is perhaps more distinct, the margins of 

 the approximating somites are seen to be compressed together, the anterior 

 margin of one with the posterior of the next, and to thin out and ultimately 

 combine together into a thin wall or plate of partition, separating the several 

 sets of muscles connected with appendages belonging to one somite from 

 those belonging to adjoining ones. Independently of being walls of separa- 

 tion thcj are points of attachment on which some of the muscles are securely 

 fixed. Not only do they exist near the lateral margin, but continue in- 

 wards and extend forwards until they reach the corresponding processes on 

 the opposite side of the percion, and also anteriorly until they unite with a 

 similar system of osseous plates in the adjoining somite. Each plate appears 

 to form a basis on which a strong muscle may take root on cither side, thus 

 forming a fulcrum for muscular power and a means of separating one set of 

 muscles from another. In Palinurus these plates, when they approximate 

 the median line, turn over and lie horizontally with the longitudinal axis of the 

 animal. These plates thus displayed form a perforated floor on which the 

 larger and more important internal viscera rest. This osseous system con- 

 tinues from the postmandibular somite persistently to the penultimate somite 

 of the percion, where it is united with the floor of the pereion by a central 

 and lateral point of contact. 



The anterior margins of the two halves of the first somite of the pereion 

 meet together in the centre and form an oblique and prominent bridge that 

 supports the posterior portion of the stomachic viscera, while the internal 

 processes of the apodema, as tkey are termed by M. Milne -Edwards, that spring 

 from the posterior two somites of the cephalon, are closely attached to, and 

 at their extremities are perfectly ossified with, the lateral and central parts 

 of the apodema of the anterior somite of the pereion, a point of union that the 

 structure of the animal requires to be of considerable strength, as the enor- 

 mous processes of the internal movable mandibular plates occupy so large a space 

 that their points of attachment necessitate a structure of greater resistance 

 and strength than the impoverished character and condition of the two 

 posterior somites of the cephalon are capable of securing to them, without 

 the additional support which they receive from a union of a more or less 

 perfect character with the anterior somite of the pereion. 



The apodema that support the internal viscera are perforated by a series 

 of foramina that, whUe they correspond in form on each side of the central 

 line, yet diff'er in size and shape according to the relative proportions of the 

 organism that are connected with them. The dimensions of the foramina, 

 through which the muscles move the large and more important appendages, are 

 larger and more conspicuous than they that relate to those that move the 

 less efficient and smaller organs of the body. Thus we find that, generally, 

 the largest and most conspicuous foramina correspond with the third somite 

 of the pereion in Palinurus, Astacus, &c., whereas in those genera where the 

 great prehensile hand is produced by the increased growth and proportions of 

 any other pair of appendages, the foramina in the apodemal plate correspond 

 with the increase of their dimensions. 



In the Anomura, of which we will take Liiliodes as the type, the internal 

 and apodemal plates do not project so as to reach the corresponding pro- 

 cesses on the opposite side. There are only six somites fused together on the 

 ventral surface, or, I should rather say, contributing to the formation of the 

 sternal plastron ; the seventh somite exists as a separate and distinct ring, 

 both dorsally and ventraUy free from ossified union with the anterior somites 

 of the pereion. 



