316 ENTOMOLOGY FOR MEDICAL OFFICERS 



Subclass Branchiopoda. 



A few Branchiopoda live in the sea and in brine-pools, but the 

 majority are inhabitants of ponds and lakes. They do not come within 

 the range of human pathology, though the smaller members of the 

 group should not be overlooked by an investigator in search of 

 intermediate hosts. Most are small ; some are microscopic ; the 

 largest attains a length of 2| inches. 



The segmentation of the body varies within wide limits, from as 

 many as 42 segments (in addition to the head) to as few as 6. Large 

 paired eyes (which may be stalked) are usually present, but may be 

 fused together. The appendages of the trunk are usually leaf-like, lobed 

 swimming-feet, which also function as gills ; hence the name of the 

 group, from /3/)ii7Xia = gills, and irois — foot. Appendages are usually 

 absent from the posterior segments. 



The sexes are separate. During the summer months, as a rule, 

 only females are found, and new generations arise from unfertilised 

 eggs ; but in the autumn generation males appear, and fertilised " winter- 

 eggs " are then produced which do not hatch until the spring. 



The eggs of Branchiopods, if they become desiccated by the drying 

 up of their pools, may retain their power of development for years, until 

 water reaches them again. This fact explains the sudden appearance 

 of Branchiopods in the rare and transient storm-pools of arid countries 

 like Baluchistan. 



Branchipus (Fig. 135) is a typical Branchiopod. Apus differs from 

 Branchipus chiefly in having the anterior two-thirds of the body enclosed 



Fio. 136. — Branchipus. 



in a large horse-shoe carapace, and in the much greater length of the 

 caudal filaments. In Estheria the body is completely enclosed in a 

 bivalve shell like that of a small fresh-water mussel. In Daphnia, which 

 is one of the smallest of the Branchiopods and consists of few segments, 

 the body is encased in a bivalve shell from which the beak-like head and 

 large antenns project. 



Subclass OSTRACODA. 



The Ostracoda (6o-rpa/<:iiS))s = shelly) are abundant both in the sea 

 and in stagnant water. They are of minute size, the largest species 

 known — a relatively gigantic form from the deep sea — being not 

 quite an inch long. The body is completely enclosed in a bivalve 

 shell, is unsegmented or very indistinctly segmented, and carries very 

 few appendages. The spermatozoa of some Ostracods " lick creation," 

 being eight or ten times as long as the animal itself. 



