330 COMPARATIVE ANATOMY CHAP. 



upper partly closed, and a lower more open, division, the lower edge of the branchio- 

 stegite being bent inwards and somewhat upwards. The reduced gills lie in the lower 

 division. The upper division holds air and functions as a lung. The integument of 

 the outer cover (branchiostegite) of this upper division carries a great number of 

 branched tufts, projecting into the air -filled cavity, and containing complicated 

 arrangements of vascular spaces. For the special relation of the vascular system to 

 the respiratory organs in Birgus, see the section on the circulatory system. 



The manner in which in the Crustacea the blood which has become poor in 

 oxygen is conducted out of the body to the respiratory organs, and the blood rich in 

 oxygen conducted out of the respiratoiy organs into the body, will be described in 

 the section on the blood-vascular system. 



II. The Integument. 



The chitinous secretion of the hypodermis (body epithelium) which 

 we met with in the Annulata is even more strongly developed in the 

 Arthropoda. It covers the whole surface of the body and its append- 

 ages as a cuticle, serving not only for protection, but also as outer 

 skeleton (exo-skeleton) for the attachment of the muscles. This chitin- 

 ous envelope is very differently constituted in the different divisions of 

 the Crustacea, sometimes even in different parts of the same animal. In 

 most Decapoda and Stomatopoda, many AmpUpoda and Isopoda, and on 

 the shell -fold of Ostracoda and Cirripedia, however, the usually thick 

 chitinous cuticle becomes very hard and firm by deposits of lime salts 

 (carbonate and phosphate of lime) ; in certain Brachyura, Cirripedia, 

 and Ostracoda it is even as hard as stone. The cuticle is, however, 

 always comparatively thin, delicate, and flexible between the joints 

 and on the respiratory surfaces. 



The constitution of the exoskeleton presents obstacles to the growth 

 of the body. These are overcome by moulting or ecdysis. Under the 

 old exoskeleton a new one is developed, which is at first soft and ex- 

 tensible ; but after the first has been thrown off it soon hardens. The 

 metamorphoses of the Crustaceans take place by means of several 

 repeated moults. In this process not only the, outer chitinous integu- 

 ment of the body, but the cuticular lining of the alimentary canal also 

 is removed and renewed. 



The cuticle, of whose further structure we cannot here speak more 

 in detail, is penetrated by fine perpendicular pores. 



Dermal glands, which take part in excretion, a fact which may be proved by 

 feeding with carmine, are very common, especially in soft-skinned Crustaceans. 



It is hardly possible as yet to give a detailed comparative account of the structure 

 and distribution of the various forms of glands. But we may give a few cases which 

 for one reason or another are specially interesting. 



The segmentally arranged ventral and leg glands of Branchipus consist of small 

 groups of dermal gland cells, found on the segments of the middle body. In every 

 segment a pair of ventral glands are found on the outer sides of the double ganglion of 

 the ventral chord, and a pair of leg glands in the basal lobes of the leg. The seg- 

 mental repetition, the character as dermal glands, the position (in the region of the 

 legs), and their rod-like secretion entirely justify the opinion that these glands are 



