968 CKUSTACEA 



essential particular to differ from the larva of Cyclops (fig. 720, C). 

 After going through a series of metamorphoses, one stage of which 

 is represented in fig. 721, B, C, these larvae come to present a form, 

 D, which reminds us strongly of that of Daphnia, the body being 

 inclosed in a shell composed of two valves, which are united along 

 the back, whilst they are free along their lower margin, where they 

 separate for the protrusion of a large and strong anterior pair of 

 prehensile limbs, provided with an adhesive sucker and hooks, and 

 of six pairs of posterior legs adapted for swimming. This bivalve 

 shell, with the members of both kinds, is subsequently thrown off; 

 the animal then attaches itself by its head, a portion of which, in 

 the barnacle, becomes excessively elongated into the ' peduncle ' of 

 attachment, whilst in Balanus it expands into a broad disc of 

 adhesion ; the first thoracic segment sends backwards a prolongation 

 which arches over the rest of the body, so as completely to inclose 

 it, and of which the exterior layer is consolidated into the ' multi- 

 valve ' shell ; whilst from the other thoracic segments are evolved 

 the six pairs of cirri, from whose peculiar character the name of 

 the group is derived. These are long, slender, many-jointed, tendril- 

 like appendages, fringed with delicate filaments covered with 

 cilia, whose action serves both to bring food to the mouth and to 

 maintain aerating currents in the water. The balani are peculiarly 

 interesting objects in the aquarium on account of the pumping 

 action of their beautiful feathery appendages, which may be watched 

 through a tank microscope ; and their cast skins, often collected by 

 the tow-net, are well worth mounting. 1 



Malacostraca. The chief points of interest to the microscopist 

 in the more highly organised forms of Crustacea are furnished by 

 the structure of the exoskeleton, and by the phenomena of meta- 

 morphosis, both which may be best studied in the commonest kinds. 

 The exoskeleton of the Decapods in its most complete form consists 

 of three strata, viz. 1, a horny structureless layer covering the 

 exterior ; 2, an areolated stratum ; and 3, a laminated tubular sub- 

 stance. The innermost and even the middle layers, however, may be 

 altogether wanting ; thus, in the larval forms known as Phyllosomata 

 or 'glass-crabs,' the envelope is formed by the transparent horny 

 layer alone ; and in many of the small crabs belonging to the genus Por- 

 tunus the whole substance of the carapace beneath the horny invest- 

 ment presents the areolated structure. It is in the large thick-shelled 

 crabs that we find the three layers most differentiated. Thus in 

 the common Cancer pagurus we may easily separate the structure- 

 less horny covering after a short maceration in dilute acid ; the 

 areolated layer, in which the pigmentary matter of the coloured 

 parts of the shell is chiefly contained, may be easily brought into 

 view by grinding away from the intwr side as flat a piece as can be 

 selected, having first cemented the outer surface to the glass slide, 

 and by examining this with a magnifying power of 250 diameters, 

 driving a strong light through it with the achromatic condenser ; 



1 Valuable details as to the structure of this group will be found in Dr. P. P. C. 

 Hoek's report on the Cirripeds collected by H.M.S. Challenger. Compare, also, 

 M. Nussbaum, Anatomische Studien, Bonn, 1890. 



