ARTHROPODA. 75 



The normal number of thoracic segments is five, but, in gene- 

 ral, two or more are fused. The abdomen is terminated by two 

 fin-shaped or setiform appendages, but is frequently rudimentary, 

 or in some reduced to its two appendages. The ovisacs are 

 attached to the last thoracic segment, where they remain, even 

 after the ovaries have parted with their contents. 



The inales are mostly rudimentary, but of many species they 

 are unknown. The young are free, and resemble Copepods ; but 

 there is a metagenesis resulting in a usually permanent attach- 

 ment to fishes, Crustacea, or mollusks, to which they adhere in 

 various ways. " Their development would seem to have been at 

 first, as it were, hurried forward at too rapid a pace, and the 

 young parasite, starting briskly into life, ranging to and fro by 

 the highest developed natatory organs we have yet met with, and 

 guiding its course by visual organs, must lose its eyes and limbs 

 before it can fulfil the destined purpose of its creation." [Owen.] 



The Epizoa are supposed to be more numerous than the whole 

 class of fishes. They are the " Crustaces suceurs " of Milne- 

 Edwards, but including also his ' Crustaces araneiformes " [Podo- 

 somata]. By many modern zoologists they are included in the 

 Copepoda, to which they are nearly allied ; but Schmarda (1878) 

 continues to keep them apart. 



Three or four pairs of limbs SIPHONOSTOMA. 



Limbs rudimentary LERN^EODEA. 



Order I. SIPHONOSTOMA. 



PARASITA. THECATA. POZCILOPODA. ONCHUNA. 

 CORMOSTOMATA. 



Body divided into head, thorax, and abdomen ; the thorax 

 segmented. Three pairs of short thoracic foot-jaws. " Two an- 

 tennae." (Milne- Edwards.) 



The head is generally confounded with the first or first two 

 thoracic segments. The antennae are two- or more jointed ; the 

 inner or lower pair are modified into hook-shaped and clasping 

 organs, and are not recognized as antennae by some authors. 

 Besides the three pairs of foot-jaws, there are four pairs of nata- 

 tory legs. In a few species the thorax is furnished with two ap- 

 pendages, resembling elytra. 



In this and the following order, the families and genera are 

 given after Glaus. Corycaeidae are often referred to Copepoda. 



