NO. 10 CRUSTACEAN METAMORPHOSES SNODGRASS 35 



Copepod fish parasites are not all content with attacking the scales, 

 fins, or gills of the host. Some make their abode in the nostrils of the 

 fish ; others penetrate through the skin into the body cavity where 

 they attaclv the vital inner organs. The worst of them are members 

 of the genus Phrixocephaliis, several species of which are described 

 by Wilson (1917). These parasites bore into the eyes of their victims 

 in order to feed from blood vessels at the back of the organs. Para- 

 sites seem to have been endowed by nature with great versatility, but 

 the life of a fish is nothing to be envied. 



CIRRIPEDIA 



The cirripeds include the familiar barnacles and several groups of 

 parasitic species. The first-stage larvae in most cases are nauplii 

 usually characterized by the presence of a pair of lateral frontal horns 

 on the anterior part of the body. In some species the horns are merely 

 short spines (figs. 14 B, 16 A, fh), in others they are long and either 

 straight or curved, but when present the horns identify the nauplius as 

 a young cirriped. The nauplius becomes a metanauplius ; the meta- 

 nauplius transforms into a free-swimming larval stage known as a 

 cypris because its body is enclosed in a bivalve shell with a closing 

 muscle, and thus resembles the ostracod of the same name. The 

 cirriped cypris (fig. 14 C) has six pairs of swimming legs, a simple 

 median eye, compound lateral eyes, and a pair of antennules project- 

 ing from the anterior end of the shell. After swimming freely for 

 some time the cypris of most species attaches itself by the antennules 

 to some solid object on which it remains permanently fixed and here 

 develops into the adult form. 



The barnacles in the adult stage (fig. 14 F, H) are sedentary on 

 rocks, clam shells, wooden piles, ship bottoms, whales, or almost any- 

 thing else in the ocean, and they get their food from the water. The 

 parasitic cirripeds attach themselves to other animals and derive their 

 sustenance from the host. The adult barnacles retain enough of their 

 ancestral structure to be recognized as crustaceans; some of the para- 

 sitic cirripeds, on the other hand, undergo such extreme degrees of 

 adult metamorphosis that their crustacean derivation is known only 

 from their early larval stages. 



The Ascothoracica. — The members of this suborder are of particu- 

 lar interest because as adults they appear to be equivalent to the cypris 

 stage of other cirripeds. If they truly are cirripeds, therefore, they 

 evidently are a primitive group of the order, and suggest that the 

 cirripeds have been derived from cyprislike ancestors, perhaps re- 



