PYCNOGONIDAE 459 



ticula extending off from the intestine into the legs and the mandibles 

 almost to their tips. The anus at the end of the abdomen. No respira- 

 tory or excretory organs are present. The nervous system consists of a 

 brain and a ventral nerve chord with five pairs of ganglia. The sexes 

 are separate, the gonads consisting of a pair of testes or ovaries which 

 are fused together in the adult and much branched. The genital pores are 

 situated in most cases on the second segment of the last two legs in the 

 male and of all the legs in the female. The eggs are carried by the males 

 on the ovigerous legs and hatch usually into six-legged larvae, which go 

 through a metamorphosis. The larva of Pallene resembles the adult at 

 hatching. 



The pycnogonids live among marine algae and on hydroids, and vary 

 in length from a millimeter to a number of centimeters. Their system- 

 atic position is very obscure, but they are usually grouped with the 

 arachnids. They differ from these animals, however, in having seven 

 pairs of appendages, a difficulty which may be overcome by considering 

 the last pair of legs abdominal and homologizing them with the first pair 

 of embryonic abdominal legs, which characterize the other arachnids. 

 The animals have been classed with the Crustacea and the larva compared 

 with the nauplius, to which it has a superficial resemblance. They have 

 also been derived from a primitive annelid ancestor and the characteristic 

 larva compared with a modified annelid larva. The order contains about 

 9 families and over 200 species. 



Key to the families of Pycnogonida here described: 



o x Mandibles wanting 1. PYCNOGONIDAE 



a a Mandibles present. 



6 t Mandibles not chelate 2. AMMOTHEIDAE 



& 2 Mandibles chelate. 



c t Pedipalps wanting 3. PALLENIDAE 



c a Pedipalps present 4. NYMPHONIDAE 



FAMILY 1. PYCNOGONIDAE. 



Body broad and flat ; mandibles and pedipalps wanting : 1 genus. 



PYCNOGONUM Briinnieh. Ovigerous legs 10-jointed, wanting in the 

 female; accessory claws wanting: 10 species. 



P. littorale (Strom) (Fig. 719). Length 16 mm.; spread of legs 38 

 mm.; eyes small, black, and widely separated, on a prominent tubercle; 

 ovigerous legs very small : Long Island Sound to Gulf of St. Lawrence, 

 from low-water mark to 400 fathoms, usually under stones; cosmo- 

 politan; Europe. 



FAMILY 2. AMMOTHEIDAE. 



Body broad and thick; mandibles not chelate; accessory claws 

 present or not: 9 genera and 30 species. 



