158 THE ANATOMY OF INVERTEBRATED ANIMALS. 



The Turbellaria possess vessels of two kinds : 1. Water- 

 vessels, which open externally by one or more pores, and are 

 ciliated. When these vessels are present, there are usually 

 two chief lateral trunks, from which many branches are given 

 off. It is probable that the ultimate ends of these branches 

 open into lacunar interspaces between the elements of the 

 tissues of the mesoderm. 2. Pseud-haemal vessels, which ap- 

 pear to form a closed system, usually consisting of one median 

 dorsal and two lateral trunks, which anastomose anteriorly 

 and posteriorly. The walls of these vessels are contractile 

 and not ciliated, and their contents are clear, and may be 

 colored. These two systems of vessels have been shown by 

 Schulze to coexist in Tetrastemma. The nervous system con- 

 sists of two ganglia placed in the anterior end of the body, 

 from which, in addition to other branches, a longitudinal cord 

 extends backward on each side of the body. In some cases, 

 these lateral trunks exhibit ganglionic enlargements, from 

 which nerves are given off ; and they may become approxi- 

 mated on the ventral side of the body, thereby showing a 

 tendency to the formation of the double ganglionated chain 

 characteristic of higher worms. Most possess eyes, and some 

 have auditory sacs. The Turbellaria are both monoecious 

 and dioecious, and the reproductive organs vary from the 

 utmost simplicity of structure to considerable complexity. 

 In most, the embryo passes by insensible gradations into the 

 form of the adult, but some undergo a remarkable metamor- 

 phosis. 



The Turbellaria are divisible into two groups. In the one, 

 the Aprocta, the digestive cavity is caecal, having no anal 

 aperture ; in the other, the JProctucha, it is provided with an 

 anal opening. The two groups form parallel series, in each 

 of which organization advances, from forms which are little 

 more than gastrulae provided with reproductive organs, to 

 animals of relatively high organization. In the simplest of 

 the Aprocta, such as Macrostomum} the oral opening is 

 devoid of any protrusible muscular proboscis, and the aliment- 

 ary sac is a simple straight bag. The male and female gen- 

 erative organs are united in the same individual, and each 

 consists of an aggregation of cells; which, in the former case, 

 gradually enlarge, fill with yelk-granules, and become ova ; 

 while, in the latter, they are converted into spermatozoa. 

 The generative cells are contained within a sac, which opens 



1 E. Van Beneden, " Recherches sur la Composition et la Signification de 

 l'CEuf," 1870, p. 64. 



