No. 376.] REVIEWS OF RECENT LITERATURE. 277 
Ludwig. Unless this assumption is made, however, the evident 
correspondence between the secondary tentacles of the two forms 
which Dr. Clark has pointed out would mean nothing, because the 
primary tentacles would not be homologous. 
This entire assumption is plausible for these reasons: (1) there is 
an evident degeneration of the water vascular system of the Synap- 
tidæ which would be expected to affect first the primary tentacles ; 
(2) the primary tentacles can more easily be supposed to have 
originally sprung from radial canals than vice versa, since the second- 
ary tentacles, in fact, are known to arise from radial canals, whereas 
there is little evidence in favor of the view of Semon that the ten- 
tacles in the primitive holothurian arose directly from the circular 
water canal. 
An abundance of anatomical and histological evidence could be 
given to show that the Synaptide are more closely related to the 
Molpadiide (e,g., Caudina) and their allies the Cucumariide than 
to any other groups of holothurians. Hence we may, as suggested 
by Ludwig, regard these three families as forming one of the two 
great branches of the family tree of the holothurians. 
While the Synaptide retain some primitive characteristics, as, for 
example, hermaphroditism, they are in many respects highly special- 
ized forms, particularly as regards sense organs. Otocysts, contain- 
ing a single vesiculated cell, and sensory papilla upon the surface 
of the body and the abaxial surface of the tentacles are described in 
S. vivipara. The ganglia found by Cuénot’ at the base of these 
papilla in S. izhaerens O. F. Müller have been observed also in S. 
vivipara. The structure of similar ganglia in S. girardii Pourtalts 
and S. roseola Verr. of our own coast has lately been investigated 
with methylene blue by the present writer. 
When the larva has still only ten tentacles a pair of “eyes ” appear 
in the connective tissue at the base of each tentacle, and later, when 
the last two accessory tentacles appear, one in each lateral dorsal 
_interradius, another pair of eyes is formed at the base of each of 
them. A knob-shaped protuberance grows out from the side of each 
tentacular nerve, where it arises from the nerve ring, into the con- 
nective tissue, and becomes covered with a layer of mesenchyme cells. 
This mesenchymatous covering in the adult consists of a “rather 
horny ” layer of light brown color, containing scattered nuclei; it is 
continuous with a thin mesoderm layer which is said to surround all the 
1 L. Cuénot, Etudes morphologiques sur les Echinodermes. Archives de Biol., 
tome xi, pp. 313-680, Pl. XXIV-XXXI. 
