CllUSTACEA. 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 oesophagus ; on each side a nervous string 

 descends a ring being thus formed round the oesophagus 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 antennae of the larvae of Cirripeds in the last form 

 contain the ducts of the cement glands, which may be traced from 

 within the discs of the antennae 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 

 antennae, 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 Lepadidce, 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 

 3d the larvae of Anatifa, without however recognising their true nature. See 

 latuurk. Verlust. Tab. vill. fig. -2, which almost entirely agrees with the figure of 

 IOMPSON Philos. Trans. 1. 1. fig. 5. Perhaps also PI. vi. fig. I. of SLABBER is a tran- 

 form of another species. 



