422 ARTHROPODA. 



Salpa}, and the NOTODELPHID^, parasitic in the gills of ascidians, form a 

 transition to the next order. 



Order II. Siphonostomata (Parasita.) 



There are also Copepoda to which the account in large type will not 

 apply, animals of such strange appearance that many of them were long 

 regarded as parasitic worms (figs. 6, 423, 425). Their mandibles are 

 altered to piercing bristles and enclosed in a piercing proboscis formed of 

 upper and lower lips. With this sucking organ they bore into the skin or 

 gills of fishes. They have cylindrical forms or bodies of the most bizarre 

 shapes, in which frequently no segmentation is visible, while the append- 

 ages are rudimentary or even entirely lost. Indeed one would not recog- 

 nize them as arthropods save for the following features : 



(1) Most of them have the typical Copepod egg-sacs (sometimes elongate 

 and spirally coiled) attached to the hinder end. (2) In the course of years 

 a complete series of intermediate forms has been found, allowing one to 

 trace, step by step, the alterations of form from that of the free-living 

 species to that of the most modified parasites. (3) Ontogeny is convincing. 

 Most parasitic Copepoda leave the egg as a nauplius and pass through a 

 Cyclops-stage before attaching themselves to fishes and becoming the highly 

 degenerate parasites. These parasites are always females. The males 

 scarcely pass the Cyclops-stage, copulate with the females 

 and then die, or if they pass through the metamorpho- 

 sis, they remain small and different in appearance. 

 They occur attached to the female near the genital 

 openings. There is thus here a marked sexual dimor- 

 phism. 



The ARGULID^E (sometimes, but without warrant, 

 made a distinct sub order, Branchiura) are fresh-water 

 forms with compound eyes, liver lobes, and the second 

 maxillipeds metamorphosed into suckers. Argulus * 

 FIG. 425. Lerncea (fig. 424) causes considerable mortality to fish. The 

 branchiaiis*>rig.) marine aud brackish-water CALIGID.E (Caligus *) have 

 similar habits. LERN^EOPODID^:. Fish parasites with maxillae united into 

 an adhesive organ. Achtheres * (fig. 6), parasitic on perch. LERN^EID^E ; 

 worm-like parasites. Lerncea branchialis,* common on gills of cod ; 

 Lernceocera* (fig. 423), on pike ; Penella.* 



Sub Class 1 V. Ostracoda. 



Like the Cladocera and the Estheriidae the Ostracoda are en- 

 closed in a bivalve shell, which, when closed, includes not only 

 the body but the head and appendages as well, these being pro- 

 truded when the shell is opened. The valves are closed by an 

 adductor muscle, opened by a hinge ligament like that of lamel- 

 libranchs. This resemblance to the molluscs is heightened by 

 lines of growth upon the shell. 



