214 



THE MICROSCOPE AND ITS REVELATIONS. 



members of both kinds, is subsequently thrown-off; the animal then 

 attaches itself by its head, a portion of which, in the Barnacle, be- 

 comes excessively elongated into the ' peduncle' of attachment, whilst 

 in Balanus it expands into a broad disk 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 exerior layer is 

 consolidated into the 'multi valve' shell; whilst from the other thoracic 

 segments are evolved the six pairs of cirrhi, from whose peculiar char- 

 acter 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 inter- 

 esting objects in the Aquarium, on account of the pumping action of their 

 beautiful feathery appendages, which may be watched through a Tank- 



FIG. 412. 



Development of Balanus balanoides;*., earliest form; B, larva after second moult; c, side 

 view of the same; D, stage immediately preceding the loss of activity; a, stomach (?); b, nucleus 

 of future attachment (?). 



Microscope; and their cast skins, often collected by the Tow-net, are 

 well worth mounting. 



613. MALACOSTRACA. The chief points of interest to the Micro- 

 scopist in the more highly organized forms of Crustacea, are furnished 

 by the structure of the shell, and. by the phenomena of metamorphosis, 

 both which may be best studied in the commonest kinds. The Shell of 

 the Decapods in its most complete form consists of three strata namely, 

 1, a horny structureless layer covering the exterior; 2, an areolated 

 stratum; and 3, a laminated tubular substance. The innermost and 

 even the middle layers, however, may be altogether wanting; thus, in 

 the PJiyllosomcB or i glass-crabs,' the envelope is formed by the trans- 

 parent horny layer alone; and in many of the small crabs belonging to 

 the genus Portuna, the whole substance of the carapace beneath the 

 horny investment presents the areolated structure. It is in the large 

 thick-shelled Crabs that WQ find the three layers most differentiated. 



