326 SYNOPTIC COLLECTION. 



Limulus of the Arachnozoa are so natural, and the bonds 

 uniting Limulus with the scorpion and spiders so close, 

 that we have placed the trilobites as the primitive Arach- 

 nozoa instead of the primitive Crustacea. 



Among the generalized living Crustacea are the marine 

 pelagic Copepods which may be taken to represent the 

 ancestral form of the group. These exist in inconceivable 

 numbers both in the tropical waters and the arctic seas. 

 Calanus (= Cetochilus) (PI. 801, figs. 1-3, C. septentri- 

 onalis Goodsir; PL 802, figs, i, 2, C. propinquus Brady), 

 begins life with few parts and organs, and the changes 

 which convert the larva into the adult are of the simplest 

 (Kingsley). The larva, called the nauplius (PI. 80 1, fig. i), 

 has a body consisting of one part, the cephalothorax, 

 which is provided with a median simple eye and three 

 pairs of appendages corresponding to the two pairs of 

 antennae and one pair of mandibles. The abdomen grows 

 out (fig. 2) and more appendages are developed (fig. 3). 

 This continues until, after several moults, the adult form 

 is attained. 



The body of the adult Calanus (PI. 802, fig. i, dorsal 

 view of female; fig. 2, side view of the same), which is 

 less than a quarter of an inch in length, is made up of 

 segments most of which are distinct and freely movable. 

 The cephalothorax is not covered by a carapace, peculiar 

 to more specialized forms, though the anterior segments 

 are more or less fused together. . The abdomen is pro- 

 vided with terminal appendages only, but the cephalothorax 

 has two pairs of antennae, four pairs of mouth parts, and 

 four or sometimes five pairs of swimming-feet. 



The anterior pair of antennae are long (fig. 2), equalling 

 the length of the body, and are adapted for a purely nata- 

 tory life. They are spread out, according to Brady 1 " at 

 right angles to the body, acting like the wings of a hover- 

 ing bird and so suspending the animal at almost perfect 



iChall. Rep., Zool., VIII, 1883, p. 30. 



