THE STTIUCTURE AND AFFINITIES OF THE SEA-SQUIllTS. 247 
late — like that of the highest Vertebrata, iiwly 'placental. The 
^‘placenta’’* (fig. 11, pi.) consists of two sacs; the outer of 
which, concave and cup-shaped, communicates with the dorsal 
sinus of the foetus; while the inner, which is spherical, and received 
into the concavity of the outer sac, opens into a sinus arising 
behind the heart of the parent. Each sac is divided internally 
by an incomplete partition, so that a current sets dowm one 
side and up the other. There is no communication hetiveen 
the sacs, the flow of blood in each being independent of that 
in the other. In process of time this bond is snapped asun- 
der, and the offspring is launched into the wide waters as 
“ Salpa A.” 
To sum up, in the words of Professor Huxley, “ There is no 
‘ alternation of generation,’ if by generation sexual generation 
be meant ; but there is an alternation of true sexual genera- 
tion with the altogether distinct process of gemmation.'’'’ 
Professor Huxley, after clearly pointing out the difference 
between a zoological and metaphysical individual,” proposes 
to term each form of Salpa — namely, A and B — a “ zooid ;” 
seeing that these are only collectively equivalent to the in- 
dividual” of higher animal forms, being in structure but part 
of an individual, namely, organs.] 
The male reproductive gland of Salpa B surrounds the intes- 
tine like a network. Since its stages of development are later 
than those of the ovary of the same zooid, it may be inferred 
that impregnation takes place from without. 
In a young A Salpa may be seen at the hinder part of the 
sinus surrounding the viscera a mass of oil-containing cells, 
termed by Krohn ‘‘ elmoblast” J (fig. 11*). The function of this 
organ is unknown ; but its occurrence in an animal possessed of 
a placental circulation suggests an analogy to a ‘^thj^mus” 
gland. 
The Salpidce are said by Chamisso to be luminous, but no 
reference is made by Huxley to this point. 
Having now described a representative both of the fixed and 
wandering Tnnicates, but small space is left at our command 
for mention of the varieties of these two groups. 
* This structure, which forcibly reminds us of oue of tlie cotyledons ” of 
the placenta of a cow, “is identical in structure with a single villus con- 
tained in a single venous jcell of the mammalian placenta, except that in the 
Salpian placenta the villus belongs to the parent, the cell to the foetus ; the 
reverse obtaining in the Mammalia.” 
t The process of generation just detailed is termed by Quatrefages, “Ge- 
neagenesis.” See cliaps. xiii. — xvi. inclusive, of his 3IetamorpIwscs of JIan 
and the Loiver Animals. Translated by TIenry Lawson, M.D. London, 
1864. 
f From the Greek tXaiov (olive) oil, we suppose. 
