THE MARINE TURBELLARIA. 



303 



Eye-spots (see Fig. 72) much as in G. rubrocincta, but the hinder margin of the eye- 

 bearing area is straighter.] 



II. NOTES AND CONCLUSIONS. 



Most of the species described above have been examined by means of serial sections, the 

 only satisfactory method of dealing with preserved specimens of this group. In some cases, 

 however, I found it practicable to diagnose species sufficiently without having recourse to sections. 

 This was especially the case with various specimens belonging to Pseudoceros. 



I have not attempted to deal at all fully with the anatomy of any specimen described in 

 the foregoing account, partly because space did not permit of my doing so and partly because 

 Lang's Monograph has rendered it unnecessary. For fuller information concerning the anatomy 

 of certain species of Planocera reference may be made to von Graff [6], and a very full account 

 of the genus Thysanozoon has been given by von Stummer-Traunfels [14]. 



Planocera and a new Genus. Lang divides the genus Planocera as defined by him 

 into two groups, A and B. These groups may briefly be defined as follows: — 



A. Five to seven pairs of branches from the main gut. Penis cylindrical, its lumen 



lined with chitinous spines ; lying in a muscular outer sheath. Female apparatus 

 provided with a muscular bursa copulatrix. 



B. Gut branches numerous. Penis styliform ; no bursa copulatrix. 



The really striking anatomical distinctions between the two groups argue equally marked 

 ■physiological differences. That such exist is, I believe, shown by the fact that Wheeler [16] 

 has proved that in PI. inquilina, a species belonging to group B, hypodermic impregnation 

 frequently occurs, in fact that in some cases the vagina may have no function other than that 

 of a passage for the escape of fertilized eggs. A similar phenomenon may occur in other members 

 of group B. On the other hand in group A the structure of the penis would preclude the 

 possibility of h}'podermic impregnation, whilst the presence of a large muscular bursa copulatrix 

 indicates that fertilization is the result of a 

 normal process of copulation. Such differences 

 are quite sufficient to warrant the raising of 

 the two groups to generic rank. The name 

 Planocera must be retained for ' group A,' whilst 

 for ' group B ' I venture to suggest the name 

 Hoploplana. 



In Fig. 73 copied from Lang the arrange- 

 ment of the genital organs of Hoploplana in- 

 signis is shown. If this figure be compared with 

 PI. XV. fig. 12, the difference between a true 

 Planocera, such as PI. armata, and Hoploplana 

 will be apparent. 



The genus Planocera then as restricted may 

 be defined in the same terras as group A is defined above, as Planoceroids with — Five to seven 

 pairs of branches from the main gut. Penis cylindrical, its lumen lined with chitinous spines; 

 lying in a muscular outer sheath. Female apparatus provided with a muscular bursa copulatrix. 



G. 39 



KS 



Fig. 73. Genital apparatus of Hoploplana inaignis (after 

 Lang). tia.= vagina, «t. = uterus, sft. 3;. = shell-gland, 

 v.s. = vesicula aeminalis, p.s. = penis, d. e. = ductus ejacu- 

 latorius. 



