ENTOMOSTRACA. 267 



colour. In Chondracanthns , as in many other cases, the para- 

 sitic females carry the pigmy males attached to their body. 

 Caligus, a very common genus of " fish-lice." 

 Lernvea, Penella, etc. The adult females are parasitic, and 

 almost worm-like. The males and the young are free. That 

 the males are often free and not degenerate, while their mates 

 are parasitic and retrogressive, may be understood by con- 

 sidering — (1) the greater vigour and activity associated with 

 maleness ; (2) the fact that parasitism affords safety and 

 abundance of nutrition to the females during the reproductive 

 period. 



Older 4. Cirripedia. — Barnacles and acorn-shells, and some allied 

 degenerate parasites. 



Marine Crustaceans, which in adult life are fixed head down- 

 wards. The body is indistinctly segmented, and is enveloped 

 in a fold of skin, usually with calcareous plates. The anterior 

 antennae are involved in the attachment ; the posterior pair are 

 rudimentary. The oral appendages are small, and in part 

 atrophied. In most there are six (or less frequently four) 

 pairs of two-branched thoracic feet, which sweep food par- 

 ticles into the depressed mouth. The abdomen is rudimentary. 

 There is no heart. The sexes are usually combined, but 

 dimorphic unisexual forms also occur. The hermaphrodite 

 individuals occasionally carry pigmy or ' ' complemental " 

 males. The spermatozoa are mobile, which is unusual among 

 Crustacea. 



Lepas, the ship-barnacle, is as an adult attached to floating logs and 

 ship-bottoms. The anterior end by which the animal fixes itself is 

 drawn out into a long flexible stalk, containing a cement gland, the 

 ovaries, etc. , and involving in its formation the first pair of antenna; and 

 the front lobe of the head. The second antenna; are lost in larval life. 

 The mouth region bears a pair of small mandibles and two pairs of 

 small maxilla;, — the last pair united into a lower lip. The thorax has 

 six pairs of two-branched appendages, and from the end of the rudi- 

 mentary abdomen a long penis projects. At the base of this lies the 

 anus. Around the body there is a fold of skin, and from this arise five 

 calcareous plates, an unpaired dorsal carina, two scuta right and left 

 anteriorly, two terga at the free posterior end. The nervous system 

 consists of a brain, an oesophageal ring, and a ventral chain of five or 

 more ganglia. There is a fused pair of rudimentary eyes. No special 

 circulatory or respiratory organs are known. Two excretory (?) tubes 

 lead from (cceloinic) cavities to the base of the second maxilla;, and are 

 probably comparable with shell-glands and with nephridia. There is a 

 complete food canal and a large digestive gland. Beside the latter lie 

 the branched testes, whose vasa deferentia unite in an ejaculatory duct 

 in the penis. From the much-branched ovaries in the stalk, the ovi- 

 ducts pass to the first thoracic legs, where they open into a cement- 

 making sac, opening to the exterior. The eggs are found in fiat cakes 

 between the external fold of skin and the body. 



The life history is most interesting. Nauplius larva; escape from the 



