CRUSTACEA. 635 
a separation into segments is indicated. It terminates backwards in 
a thin tail, which earlier writers improperly called the proboscis. 
Along the ventral surface there are six pairs of arms, each of which 
bears on a short pedicle two long filaments turned inwards, that 
consist of numerous joints and are beset with hairs. The animals 
extend these filaments constantly during life, from the opening of 
the shell and draw them into it again, by which they introduce 
within the shell the water for respiration and with it their nutri- 
ment. 
The nervous system consists of a row of six ganglia, or properly 
pairs of ganglia, connected by two nervous cords. The first ganglion 
is situated on the short cesophagus ; on each side a nervous string 
descends—a ring being thus formed round the cesophagus—to the 
second ganglion, which is situated between the first pair of feet. The 
intestinal canal is straight and very short ; it runs along the dorsal 
surface and terminates at the base of the tail. Through this tail 
runs the common efferent tube of the two vasa deferentia and opens 
at its point. These animals are bisexual, with some exceptions (see 
p- 608). 
[The prehensile antennz of the larvee of Cirripeds in the last form 
contain the ducts of the cement glands, which may be traced from 
within the discs of the antennz to the anterior or lower ends of the 
incipient ovaria. The gland which secretes the cement appears to 
be a part of an ovarian tube specially modified. If the base of at- 
tachment of a Cirriped be carefully removed, the larval prehensile 
antenne, from which the cement always escapes, will be found very 
near to its extremity. In sessile Cirripeds a new cement gland is 
formed at every period of growth. Darwin Lepadida, 28, 33'.] 
Cirripeds are found in the seas of every region of the world ; they 
attach themselves to rocks, to many marine animals as shell-fish, 
turtles, whales, to different polyparies, to marine plants, to the hulls 
of ships, to the floating wreck of vessels that have been lost, as frag- 
ments of wood, bottles, &c. 
1 Our countryman SLABBER, as I lately discovered, was the first who observed and 
figured the larve of Anatifa, without however recognising their true nature. See 
Natuurk. Verlust. Tab. vit. fig. 2, which almost entirely agrees with the figure of 
Tuompson Philos. Trans. 1. 1. fig. 5. Perhaps also Pl. vi. fig. 1. of SLABBER is a tran- 
sitory form of another species. ~ 
