CIRCULATION. SEXUAL ORGANS. DEVELOPMENT. 40I 



The Heart and Circulation. — In the acraniates the heart and the vascular 

 channels are feebly developed and may be altogether absent or unrecognizable. 

 There is no definite vascular system in the polyzoa and chaetognatha; and a distinct 

 heart is absent in cirripeds, Amphioxus, the phoronida, and the echinoderms; 

 it exists, if at all, in a highly modified condition in the enteropneusta and ptero- 

 branchia. In the tunicates and brachiopods a small, oval heart is present. In 

 the ascidians it is a small fusiform tube, unsegmented and valveless, and composed 

 of epithelio-muscular cells. Its cavity is derived from the so-called blastocoele 

 and it is enclosed in a pericardium derived from the coelom. In these respects, 

 and in respect to its location, the distribution of the principal blood channels, 

 and in its mode of development, it resembles the "heart" of phyllopods. The 

 tunicate heart is notable for its "reversing circulation," that has been regarded as 

 something unique in the animal kingdom; but a similar phenomenon has been ob- 

 served by Scott, in the parasitic- copepod Lepeophtheirus. See page 418. 



The Sexual Organs. — Both ovaries and testis may be present in the same 

 individual. In the cirripeds, the testis usually occupies the posterior part of the 

 trunk, opening to the exterior at the apex of the caudal lobe. The ovaries are 

 lodged in the cephalic region, extending also into the peduncle and mantle folds, 

 and even into the recesses of the exoskeleton. The oviducts open outward near 

 the middle of the body, at the base of the first pair of abdominal cirri. (Fig. 275.) 

 In the tunicates a similar condition may prevail, e.g., in Polycarpa, where "there 

 are many complete sets of both male and female systems attached to the inner 

 surface of the mantle, on both sides of the body." Moreover embryonic "kalym- 

 mocytes," or egg follicle cells, frequently pass through the ectoderm into the cellu- 

 lose test, suggesting a former connection with the mantle, like that in cirripeds. 



In the brachiopoda there are two pairs of genital glands, both pairs located in 

 the mantle, one in the anterior fold, the other in the posterior. The genital cells 

 are usually discharged by a pair of nephridia-like ducts that open on the neural 

 surface near the middle of the body, or just behind what appears to represent the 

 thoracic region; chaetognatha, pterobranchia, polyzoa, brachiopods, phoronida. 



The germ cells of arthropods may make their appearance as the so-called 

 pole cells at a very early period, before any germ layers are recognizable. In 

 parasitic copepods, they arise from the undifferentiated blastoderm, and later 

 form a small but conspicuous cluster of cells on the neural surface, between 

 the abdominal and thoracic neuromeres. (Fig. 242.) In the chaetognatha and 

 polyzoa, the germ cells are conspicuous at an early period in a corresponding 

 position. (Figs. 301, 306.) 



Development. 



It will be necessary to abandon, or greatly modify some deep-rooted con- 

 ceptions as to the significance of the germ layers and early embryonic processes 

 in segmented animals, for they are based either on errors of observation, or upon 

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