FRESH-WATER PLANARIAN 163 



each of these testes is connected a tube of thin epitheUal cells called a 

 vas efferens. All of these vasa efferentia join a large lateral tube on each 

 side of the body known as a vas deferens which leads to a muscular 

 cirrus, or penis, situated toward the posterior end of the body near the 

 ventral side and behind the pharynx. A seminal vesicle, in which the 

 sperm cells are accumulated, lies at the base of the cirrus. A number 

 of prostate glands pour their secretion into th'ese passages, the function 

 of their secretion being to stimulate the sperm cells to activity. The 

 sperm cells pass from the testes down the vasa efferentia and vasa defer- 

 entia and are transferred by the cirrus, in packets called spermatophores, 

 to the uterus, either of the same or of another individual. In the uterus 

 they are stored until needed. 



The female reproductive organs consist of two ovaries and two 

 tubular oviducts, one on each side, through which the egg cells pass 

 backward. Yolk glands scattered along the course of the oviducts add 

 their secretion to the egg cells as they pass, and a vagina transmits these 

 cells to a genital atrium, a chamber which receives the opening of the 

 cirrus. The pouchUke uterus is also connected with this genital atrium. 

 The term uterus is applied to that portion of the oviduct in which the 

 eggs accumulate and in which a part of the development of the embryo 

 takes place. The genital atrium opens by a genital pore on the ventral 

 surface behind the mouth. 



The body is covered by a simple cihated epithelium attached to a 

 basement membrane. There are no structures corresponding to cir- 

 culatory, respiratory, or skeletal systems. It is interesting to note that 

 in the development of systems the reproductive system, upon which 

 the perpetuation of the race depends, is in the lead, followed by the 

 digestive system, necessary to the existence of the individual and the 

 nervous system, which puts the animal in touch with its environment. 



189. Metabolism. — The food of a planarian consists mostly of animal 

 matter, though a little plant food is taken. The animal secures the 

 food by means of its proboscis, through which ingestion takes place. 

 Digestion, as in the case of the coelenterates and ctenophores, is both 

 intracellular and extracellular. The food is distributed to all parts of 

 the body by the much branched gastrovascular cavity, the wall of which 

 serves everywhere for absorption. Circulation occurs by the passing 

 of the absorbed food from cell to cell and through the spaces between 

 the cells. Elimination takes place by means of the excretory system, 

 and egestion is through the mouth. Respiration goes on over the entire 

 surface, there being no structures developed for this purpose, though 

 some authors attribute to the excretory system an expiratory function. 



190. Reproduction. — This animal reproduces asexually as well as 

 sexually. Asexual reproduction is usually by transverse fission. 



