Sub-Glass 1. Entomostraca. Order 6. Copepoda, 



203 



is a sac without limbs (Pig. 165 C), and with only two longer or 

 shorter (often thread-like) egg-sacs. It is immovably attached to 

 the host, in some cases by means of the second antennee, the second 

 maxillae or the maxillipeds modified into long arms; in others by 

 means of the whole front part of the animal, whic-h is imbedded in 

 the body of the host. The males of the more strongly modified 

 forms are usually pigmy, attain only a small fraction of the size 

 of the females, and as a rule are attached to them in the neighbourhood 

 of the genital aperture ; they are not usually so entirely modified as 

 the females may be, as a rule they have several pairs of limbs, etc. 



The parasitic Copepods, like the free-living ones, are hatched as 

 n aup lii, which swim freely about, and after some moults reach a 

 state like that of the free-living adult. The parasites owe their ultimate 

 deformity to a "retrograde metamorphosis " occurring after fixation. 



In some parasitic Crustacea, e.g., Lernsea hranchialis, living on the gills of tlie 

 Cod, the male and female are fairly alike up to the time of copulation, presenting 

 a tolerahly normal copepod form ; after pairing, however, the female grows con- 

 siderably, and becomes quite distorted, whilst the male perishes ; wherefore no 

 male is found with the adult female Lernsea. 



Order 7. Cirripedia. 



The Cirripeds are furnished with a sort of protecting shield, the 

 so-called mantle, which is attached to the rest of the animal only 



Fig. 166. Fig. 167. 



oa- 



Fig. 166. Lepas. The right half of the mantle is removed, the body shown in longi- 

 tudinal section. — ^After Claua. 



Fig. 167. Balanus. The right half of the mantle and shell taken away. — After Darwin. 

 a and b the paired valves of the mantle, u scutnm, b tergum, c unpaired dorsal valve, 

 carina. Oj anterior antennae, an anus, h cement gland, I liver, m adductor muscle of 

 mantle, m' retractor muscle, o' female aperture, od oviduct, ov ovary, p penis, r shell, 

 si vaa deferens, ( testis. 



