PHYLLOPODA. 



287 



In the family Apodidw, containing the genera Apus and Lepidurus, the anterior 

 end of the body is covered with a carapace, projecting from the head over the free 

 segments of the thorax. The hinder border of this 



o 



carapace is deeply cut out, and near its front end there is 

 a pair of contiguous compound eyes. The mouth is 

 bounded in front by a large upper lip and behind by a 

 deeply cleft metastoma, or lower lip. Both pairs of 

 antennae are short. The jaws consist of a pair of 

 mandibles and two pairs of maxilla? ; these are followed 

 by eleven pairs of thoracic limbs, and there are append- 

 ages on the abdomen, sometimes numbering as many as 

 fifty-two pairs. The last segment of the abdomen bears 

 a pair of long filaments, and sometimes, as in Lepidurus, 

 a distinct caudal plate. These crustaceans occur in the 

 fresh waters of most countries. They swim on their 

 backs, using the legs as paddles ; and the eggs are capable 

 of surviving long periods of drought when embedded in 

 dried mud. In the second family the Brancliipodidce 

 the body is also elongate, but there are no appendages 

 to the abdomen, which consists of nine segments, while 

 there are eleven pairs of thoracic appendages. The head- 

 shield is not developed backwards, and the large separated 

 eyes are supported upon distinct stalks. In the male, the 

 second antennas are converted into claspers. These forms 

 likewise swim upside down. Some (Branchipus) occur in 

 fresh waters, but others (Artemia) prefer briny pools and 

 flourish in water so strongly charged with salt as to be 



fatal to Other Crustaceans. Artemia salina (enlarged). 



MALE OF ran( .hi pus 



(nat. size) ; 6, FEMALE OF 



Class Prototracheata. 



This term is applied to the group now claiming attention, because in many 

 respects it occupies a place between the Tracheates and Worms, and is consequently 

 regarded as allied to the ancestral form from which all Tracheates have been 

 evolved. Unlike the true Arthropods, the limbs are not jointed ; and the tough 

 integument is covered with bristle - bearing papilla, but is not divisible into 

 segments. The long body is shaped like that of a caterpillar or slug, and to the 

 sides of its lower surface are attached a number of short more or less conical legs, 

 each tipped with a pair of strong claws. The head is supplied with a pair of stout 

 longish antennas, at the base of each of which, on the outer side, is an eye. On 

 the lower surface of the head is placed the mouth, supplied with fleshy lips and 

 two pairs of toothed horny jaws ; and on each side of the head there is a modified 

 appendage known as the oral papilla. The chief features to note in the internal 

 organisation are the presence of segmentally arranged kidneys one opening at the 

 base of each leg and the wide separation of the two strands of the ventral nerve- 

 chord. This last character is found in some of the lower worms, and the numbers 



