THE TURBLLLARIA 21 



exists. Geddes (19) described certain structures under the name 

 " pulsatellae," which Yves Delage (12) regards as isolated flame 

 cells ; but these have not been described by more recent observers. 



The generative organs of the Turbellaria, as in all Platyhelmia, 

 are very complicated; but they present more variations in detail 

 in the Rhabdocoelida than in the other orders, and a given plan 

 does not hold even within the same family. The male organs 

 consist of a pair l of testes, from each of which a sperm duct passes 

 backwards to open into a seminal vesicle, whence a duct perforates 

 a glandular and muscular organ or penis, which is frequently 

 armed with chitinous spines or a chitinous sheath, the character 

 of which is of generic and specific value. The penis opens into 

 an epiblastic sac known as the " atrium genitale," if it is common 

 to the male and female organs, or the duct of each sex may have 

 its own " antrum " and external pore. We have either a monogono- 

 porous (Fig. III. 3) or a digonoporous condition in the Rhabdo- 

 coelida, and the female pore may lie in front of the male pore 

 (Fig. III. 2). The genital pore was first recognised as such by 

 Johnson (36) in Planaria twva ; previously it had been regarded 

 as the anus. The penis appears to be used as much in catching 

 prey as in copulation ; and in Macrorhynchus helgolandicus, in which 

 a poison gland traverses the organ, it appears to be entirely used 

 for this purpose ; and no doubt the arrangement in Prorhynchus, 

 where the penis opens at the same pore as the pharynx, and is 

 armed with a perforated spine, has come about by the employment 

 of this organ in catching prey (32 and 39). 



In the sub-order Rhabdocoela the testis of each side retains 

 the ancestral condition of a " compact," tubular organ (Fig. VIII.) ; 

 but in the Alloiocoela and Acoela this single testis becomes con- 

 stricted (? by dorso-ventral muscles) into a number of " follicles," 

 which lie near the anterior end of the animal, each follicle of which 

 ontogenetically is derived from a single cell. Despite the most care- 

 ful recent research, the sperm duct has not been traced up to each 

 one of these follicles, and it is uncertain how the spermatozoa pass 

 from them to the seminal vesicle. One of two methods has been 

 suggested : (a) that by the enlargement of the follicles they come 

 to open into one another, and so communicate ultimately with 

 the sperm duct ; or (b) they burst into the parenchymal lacunae, 

 and their contents thus gain the duct. In some cases (Acoela) 

 the duct is not even continuous with the seminal vesicle. 



With regard to the female organs, there can be little doubt 

 but that in the primitive gonad egg cells were formed and supplied 

 with yolk by their neighbours ; such an organ or " ovary " occurs 

 in Acoela (Fig. VIII.) and in many Rhabdocoela as a single or 



1 Haswell (32) states that in Prorhynchus sp. the male organs are only on the 

 right side, and the female only on the left side, unpaired in each case. 



