DEVELOPMENT OF ROTIFERS. 137 
constant as in Calyptrea. The tail (f) becomes conical, 
lurger, and the termination of the intestine and anal open- 
ing is formed at the base. 
The internal organs are then elaborated ; first the nervous 
system, consisting of but a single pair of ganglia arising 
from the outer germ-layer (ectoderm). Soon after the sen- 
sitive hairs arise on the wings of the velum. 
Fig. 93 represents the advanced embryo, with the body di- 
vided into segments, the pair of ciliated wings of the veluin 
(v), and the long tail (¢). At this time the shell begins to 
form, and afterwards covers the whole trunk, but not the head. 
The inner organs are developed from the inner germ-layer 
{endoderm), which divides into three layers, one forming the 
middle part of the intestine, and the two others the glands 
and ovaries. The pharyngeal jaws arise as 
two small projections on the sides of the 
primitive cavity. The male develops in 
the same mode as the female. 
Though the development of the Rotifers, 
so far as known, is more like that of the 
mollusks than true worms, the Rotifers 
may be regarded as a generalized cephalula 
form, representing the larval forms of An- 
‘ . a en Fig. 93.—Brachion 
nelids and mollusks, with decided affinities, neatly ready to hatch. 
‘i Bae F ay ky. 
when we consider their chitinous covering —Af*" Salensky 
or carapace, the fold of the intestine, and the single nervous 
ganglion, to the Polyzoa, and with more remote resemblances 
to the Brachiopods. They are on the whole generalized forms. 
A few species are parasitic: Aldertia living internally, and 
Balatro on the surface of the Nais-like worms. With the 
lower Rotifers are associated a group of worm-like forms 
represented by Chetonotus, Ichthydium, ete., and forming 
the group Gastrotricha. They have no mastax, and the body 
is only ciliated near the end. Through Dinophilus, a Tur- 
bellarian worm, they are connected with the flat worms. 
The genus Zchinoderes is also regarded by Claus as a low 
Rotifer. It seems quite apparent from this that the Rotifers 
are a type which has originated from worms resembling the 
generalized Turbellarian form, and wich connects the latter 
