PORIFERA : MESOZOA. 813 



Affinities of Porifera ; see for most recent discussion, Heider on Oscarella, Arb. 

 Zool. Inst. Wien, vi. p. 41 of paper or p. 215 of vol. ; Vosmaer, 'Porifera/ pp. 472- 

 481 ; and for the late Prof. Balfour's views, his Comp. Embryology, i. p. 122; ii. 

 p. 285 ; of the Myxospongiae (pligosilicina), von Lendenfeld, Z. A. viii. 1884 ; of the 

 Monadinellidae (e.g. Suberitidae, &c.), Id. Z. A. vii. 1884; cf. viii. p. 484; of 

 Keratosa=; Ceratina, Potejaeff, Challenger Reports, xi. p. 75 et seqq. 



MESOZOA (E. van Beneden). 



Multicellular animals, in which there is only an ectoderm and endoderm 

 or epi- and hypo-blast. 



The ectoderm is composed of a single layer of cells, totally or partially ciliated, 

 the endoderm of a single cell or of several cells. There is no mesoglaea nor meso- 

 blast. The sexes are separate, and the sexual products originate from the 

 endoderm. E. van Beneden adds that 'there are two forms of -female, one pro- 

 ducing females alone, the other males ; ' and that ' the Mesozoa actually known are 

 all parasites.' The first of these two assertions does not apply to the Rhombozoa, 

 according to Whitman ; the second points to the possibility of the known Mesozoa 

 being degenerate Metazoa. 



There are two subdivisions of Mesozoa, according to the Belgian naturalist, the 

 Orthonectida and Rhombozoa. 



The Orthonectida are defined as follows by E. van Beneden : ' Body composed 

 of several annuli ; endoderm formed of several cells, some of which are epithelio- 

 muscle cells, and give origin to muscular fibres, whilst the remainder are converted 

 into sexual products. The male is elongated, annulated, and one of its anterior 

 rings is papillate. Females oviparous ' and, it may be added, dimorphic. 



There is a single genus, Rhopalura, with two species; one, Rh. Giardi, parasitic 

 in the genital bursae of the Ophiurid Amphiura squamata (=Ophiocoma negleda) 

 the other, Rh. Intoshii, in the Nemertean Lineus lacteus. Both species are there- 

 fore marine. 



Rh. Giardi has been recently investigated by Julin. The males and females 

 are usually to be found in different individuals of the Ophiurid. The parasites 

 cause an atrophy of their host's genitalia. The males are contained in saccules, ap- 

 parently produced by the host, in which they swim freely, and are, when adult, about 

 104 mm. long. Their body is fusiform and divided usually by five furrows into six 

 annuli. Each annulus is composed of a single layer of ectoderm cells. The first 

 or head is constituted by 4 or 8 cells : the second of five rows of small cells, the 

 so-called papillae, all of which are non-ciliate. The third annulus is usually the 

 longest, and contains one row of cells. The last or caudal annulus has two rows of 

 4 cells, and when the rows are very distinct, it may be considered as split into two 

 annuli. The cilia of the head are directed forwards ; of the third and following 

 annuli backwards. The caudal cilia are long and stout. At the level of the third 



