8i4 THE ANIMAL KINGDOM. 



annulus is contained the testis formed from the central endoderm cells. The sper- 

 matozoa possess a head and tail. Fibrils stretch from either end of the body 

 beneath the ectoderm over the testis ; nuclei have been detected in them. The 

 ripe sperm ruptures the testis : the fibrils (supra] thereupon become collected into 

 bundles, the ectoderm disorganised, its cells swelling and separating, while the 

 sperm escapes into the water. The females occur in two forms, one cylindrical, 

 which produces only male embryoes, the other flattened but pointed at each end, 

 from which only female embryoes originate. Both forms occur side by side. The 

 first-mentioned is -280 mm. long. It has eight annuli : the second annulus which, 

 like the third and fifth, contains only one row of cells, is non-ciliate in the adult. 

 The remaining annuli vary in the number of their rows of cells. There is a sub- 

 ectodermic fibrillar layer. The central mass consists of ova, polyhedric from mutual 

 pressure, but free from one another when they are discharged into the water by the 

 rupture of the body in the furrow behind the second annulus. The flattened female 

 is -250 mm. long. It is completely ciliated, and the cilia of the anterior and pos- 

 terior regions are directed as in the male and cylindrical female. Furrows are 

 usually wanting, and the ectoderm cells are not distinguishable during life. Sub- 

 ectodermic fibrillae are present, and near the anterior extremity of the body there is 

 a uni-nucleate granular mass (the sub-polar cells of Metschnikoff) projecting inwards 

 from the ectoderm. The ova are held together by an intervening granular sub- 

 stance. This female appears to break into segments, the Plasmodiumschlauche of 

 Metschnikoff, which swim about and contain the developing and developed female 

 offspring. 



The male ovum divides into two blastomeres; one large, the endoderm cell; the 

 other small, which divides in turn repeatedly, grows round the endoderm cell, and 

 forms the ectoderm. The endoderm cell divides into three, a central and two polar, 

 one anterior, the other posterior. The two polar cells divide in their turn and form 

 small ' intermediate cell-masses/ from which the muscle-fibrils are derived. The 

 central cell also divides and gives origin to the sperm. The youngest female 

 observed consists of a mass of endoderm cells, at first partially, then wholly sur- 

 rounded by ectoderm cells. The muscle-fibrils are produced from a superficial layer 

 of endoderm cells. It may be observed that in both sexes there is an epibolic Gas- 

 trula. The young Rhopalura is at first entirely ciliated, and the cilia are sub- 

 sequently lost from the second annulus. The five rows of papillae in the male are 

 produced by the division of two rows of ectoderm cells. 



Rh. Intoshii is very similar to Rh. Giardi^ but it is smaller. The male has no 

 papillate annulus, nor has either sex a non-ciliated annulus. The cilia of the head 

 are directed forwards, of the other annuli backwards. This species is found in sac- 

 cules (Plasmodiumschlauche) between the body-walls and digestive tract of its 

 Nemertean host. They are probably formed by the host, in which the genitalia, 

 owing to the presence of the parasite, appear either to remain undeveloped, or to 

 atrophy. The saccules may contain only males, or females, or both together, and in 

 variable numbers. Metschnikoff does not seem to have observed a second form of 

 female. It is uncertain whether Rh. Intoshii is or is not identical with the Ortho- 

 nectidan observed by Mclntosh in the same Nemertean, or by Keferstein in the 

 Polyclad Turbellarian Leptoplana tremellaris. 



The Rhombozoa are defined as follows by E. van Beneden : ' Body never an- 

 nulated ; endoderm a single cell ; no muscular fibres ; the germs originate and 



