THE BRANCHIOPODA 43 



of one and a half turns (Fig. 26). As a rule, the anterior part of 

 the mid-gut gives off a pair of diverticula. In the Notostr'aca and 

 Conchostraca these are much ramified, and in the Anostraca they 

 are saccular and lobulated. In the Cladocera they are often absent, 

 i\nd when present are simple caeca. In some Cladocera of the 

 family Lynceidae the posterior part of the alimentary canal gives 

 off an unpaired diverticulum on the ventral side (Fig. 26). It 

 is usually short, but sometimes it is of considerable length. It is 

 stated to arise from the hind -gut, and its walls contain large 

 gland-cells. 



Circulatory System. Except in the Cladocera, the heart is 

 elongated and tubular. In the Anostraca it traverses almost 

 the whole length of the trunk, and has paired ostia in each somite 

 except the first and last. In the Notostraca the heart extends 

 through the first eleven trunk-somites and has eleven pairs of ostia. 

 In the Conchostraca it is restricted to three or four somites, with 

 a corresponding number of ostia.- In the Cladocera the heart is 

 greatly abbreviated, forming a sub-globular sac with a single pair of 

 ostia, lying in the region of the first trunk-somite (Fig. 18, A). 

 There are no distinct vessels and the blood is discharged directly 

 from the anterior end of the heart (through three openings in the 

 Notostraca) into the sinuses of the head-region. 



In some genera (Apm, Branchipus, Artemia, and some Cladocera) 

 the fluid of the blood is coloured red, owing, as Lankester has 

 shown, to the presence of haemoglobin. 



Excretory System. The maxillary gland (Fig. 18, gl) is well 

 developed in all Branchiopoda. Except in the Anostraca and some 

 aberrant Cladocera (Leptodom), it lies within the "thickness of the 

 shell-fold. It shows the typical structure, consisting of end-sac, 

 glandular coiled tube, and short terminal duct. In the Notostraca, 

 as already mentioned, the external opening is at the end of a 

 tubular process arising from the under side of the head, close to 

 the maxilla. 



The antennal gland is well developed in the larval stages of 

 Anostraca, Notostraca, and Conchostraca (Fig. 10). A vestige of 

 it persists in the adult Artemia. 



Glands. Belonging to the dermal series of glands are the 

 segmentally arranged ventral and leg glands of some Anostraca, 

 and the groups of unicellular glands in the labrum which are 

 probably always present. 



Special interest attaches to the structure variously referred to 

 as " neck gland," " dorsal organ," or " adhesive organ," found in 

 many Branchiopoda. In its simplest form this consists of a 

 thickened and glandular area of the ectoderm, on the dorsal surface 

 near the posterior limit of the head-region. The adhesive secretion 

 reaches the exterior through pores or slits in the cuticle, and is used 



