THE ORTHONECTIDA. 
235 
very much elongated in a radial direction (PI. XXII, figs. 
8 to 13). This hlastula very soon exhibits a very definite 
process of delamination ; the internal part of each cylin- 
drical cell gives rise to a spherical cellular mass, which 
detaches itself from the mother-cell, which is shortened by 
the process (PI. XXII, fig. 10). This is absolutely the 
same mode of formation as that which can be observed in 
the production of the mesoderm in the Ophiurids with a 
condensed embryogeny {Ophiothrix fragilis, Ophiocoma 
neglecta). In the present case it is the endoderm which 
forms at the expense of the ectoderm, but we have seen 
that this endoderm contains potentially the mesodermic 
elements (the muscular bands). 
Soon the ectodermal cells become covered with vibratile 
cilia, and elongate in a definite direction ; then the embryo, 
which was spherical, becomes ovoid, the metameres show 
themselves little by little, and by insensible gradations we 
arrive at the adult condition. 
VII. — System ATI c. 
According to what has been stated in the preceding pages, 
the Orthonectida may be defined in the following manner : — 
Metazoa retaining throughout their existence the form of 
the planula, having a ciliated ectoderm (stiff cilia forming a 
bunch on the anterior cephalic region, vibratile cilia on the 
other regions of the body), presenting metameres, which do 
not correspond to any internal division of the body, with 
a sacciform endoderm, which gives rise to a muscular 
splanchno- pleural pseudo- mesoderm. Reproduction of two 
kinds : (1) gemmiparous in the interior of sporocysts, con- 
stituted by the development of the endoderm ; (2) oviparous, 
dependent on the combination of male and female elements, 
formed in all probability in different individuals.” 
The class comprises at present two genera : 
I. The genus Rhopalura, characterised by the presence of 
a papilliferous ring, by its ectoderm composed of large cells 
limited in number, and its definitely muscular endoderm. » 
Species unique. Rhopalura ophiocoma. Length 0T08 mm. 
II. The genus Litoshiay devoid of papilliferous ring ; 
ectoderm formed of small, very numerous cells. 
Three species : 
1. Intoshia gigas, — Parasitic in Ophiocoma neglecta. 
Length 0 270 mm. 
2. Intoshia linei, — Lineus gesserensis. Length 
0‘157 mm. 
