370 MOLLUSCA. 
Mollusca. the latter being upwards for the two lower rows, and down- papilla. No liver. A cylindrical rib extending from the Molluses. 
“=~ wards for the two upper ones. pylorus to the anus. ae 
Another larger portion of the blood, on leaving the heart, © (A.) Tunic and branchial cavity straight. 
immediately divided into many ramifications that spread like Gents Prrena.—tThe branchial sac as extended as the 
a network over the stomach and intestines and the soft sub- tunic. Stomach not resting on the intestine. P. phusca of 
stance of the mantle. Of these a part run into the horizon- 
tal passages above the branchial sac, a part into the descend- 
ing back stream ; a large portion, after leaving the intestines, 
took a short course, and, collecting into one channel, flowed 
into that stream near the bottom, and, all united, then 
entered the peduncle and constituted the returning current 
that went to circulate in other animals of the group. 
After this circulation had gone on for a while the pulsa- 
tions became fainter for a few beats, and the flow slower, and 
suddenly, with but a slight pause, the whole current in all 
its windings was reversed. The heart gave the opposite 
impulse; the channel in the peduncle, that before poured 
in the blood, now carried it back, and the other the con- 
trary, and every artery became a vein. These changes 
continued and succeed each other alternately, the average 
time of the currents being the same in both directions, but 
the period of each varying within a single observation as 
much as from thirty seconds to two minutes. The pheno- 
menon, like the currents in the Sertwlariae, was invariably 
met with in every animal of the species that came under my 
notice. 
Sometimes, when the creeping tube or the peduncle has 
been injured, the circulation of an individual is in conse- 
quence insulated, but without appearing to impair any of its 
functions. I severed one at the part where it joined the pe- 
duncle, when for a few seconds the pulsation ceased; it then 
began irregularly and with considerable pauses, and increased 
in steadiness as it went on. At first the impulse given by the 
heart was towards the front, and the downward back stream, 
instead of flowing out at the wound, was poured into the 
hinder end of the ventricle ; but when the current was re- 
versed part of the blood was driven for a time through the 
stump of the peduncle into the water: however, it soon 
staunched, and all the vital actions went on as before the 
separation, except that at the beginning of every pulsation 
there was a slight recoil. 
In one case where the circulation did not extend to an- 
other animal, one channel, and only one, was open in the 
peduncle, and in this a small current ran to and fro accord- 
inz to the direction of the impulse given by the heart. 
Some animals, which had probably been injured, but were 
still connected with other vigorous ones, seemed to be in 
course of absorption. One was observed in which the soft 
parts were so shrunk as to occupy a small part only of the 
tunic; the currents of its peduncle extended into this mass, 
but the heart, or motion of branchiae, was visible. Upon 
looking at the same the next day, the tunic was empty, the 
soft matter and circulation reaching only to the end of the 
peduncle. I also once noticed a flux and reflux of the blood 
in a creeping stem, where the current did not communicate 
with any animal. 
In some of the last mentioned particulars this Ascidia 
bears a resemblance to the Sertulariae, and, like them, 
it increases by sprouts: the two streams of the stem run 
through the bud before its organs are developed. No pro- 
per motion was seen in the particles of its blood, like that 
of the Sertulariae. 
In a sessile Ascidia, nearly half an inch in length, of 
which the coat was too rough and opake to allow an inspec- 
tion of the branchiae, the circulation was distinctly visible 
in the mantle near the openings, and the particles in the 
blood were only of about the same size as above. 
4. Body sessile. 
The branchial orifice with eight or nine rays, and the 
anal with six. The angles of the branchial meshes with 
Forskael is the type, to which Savigny has added three other 
species. The Ascidia prunum of Miiller, a native species, 
may be referred to this genus. 
Genus Crona.—Branchial sac shorter than the tunic, and 
exceeded by the viscera. Ascidia intestinalis, Lin. is a 
native example of this genus. 
(B.) Tunic turned up at the base. 
Genus Puaxiusta.—Branchial sac extending beyond 
the viscera into the pouch of the sac. Stomach resting on 
the mass of viscera. The Ascidia mentula of Miiller, a na- 
tive species, is the type. 
There are two genera supposed to be nearly related to 
the preceding, which are involved in great obscurity. The 
genus Bipapilaria of Lamark appears to be pedunculated, 
with two apertures, each furnished with three setaceous 
tentacula. The Mammaria of Miiller has only one termi- 
nal aperture. One species inhabits the British seas. 
B. Compound. 
The animals included under this division were formerly 
inserted in the genus Alcyonium of Linnzus, and placed 
among the Zoophytes. They are compound animals, many 
individuals united by a compound integument, and arranged 
according to a uniform plan. 
In some cases, there is only one system of individuals in 
the mass, in other cases, there are many similarly arranged 
and contiguous. The tentacular filaments of the branchiz 
appear to be distinct. They are destitute of the intestinal 
rib which occurs in some of the preceding genera. 
1. Branchial Orifice Radiated. 
a. Branchial and anal orifices, with six rays. 
(A.) Body sessile. The angles of the branchial meshes 
furnished with papilla. The thorax, or cavity containing 
the branchiz, cylindrical. The abdomen is inferior, with a 
stalk. Ovarium sessile, and single. 
Gents Drazona.—Body orbicular, with a single system 
of animals disposed in concentric cirles. 
The substance is gelatinous. The ovarium enclosed in 
fold of the intestine. D. violacea of Sav. tab. ii. f. 3. 
Genus Potyzona.—Body polymorphous, with many sys- 
tems disposed subcircularly. 
The body is subcartilaginous. The individuals are dis- 
posed irregularly around the common centre. Savigny 
inadvertently termed this genus Distoma, a name long 
preoccupied amongst the intestinal worms. The Aleyontum 
rubrum of Plancus, and the Distomus variolosus of Gaert- 
ner, belong to this genus. The last is a native species. 
(B.) Body pedunculated. 
Genus S1eittivna.—Body a solid cone, consisting of a 
single system of many individuals, irregularly disposed, one 
above the other. 
The thorax is short, and hemispherical. The angles of 
the branchial meshes destitute of papillae. The abdomen 
is inferior, sessile, and larger than the thorax. The single 
ovarium is pedunculated. S. astralius, Sav. tab. iii. f. 2., 
brought from New Holland, by M. Peron, is the only known 
species. 
6. Branchial orifice only furnished with six rays. 
(A.) Body pedunculated. System single, circular, and 
terminal. 
Genus Synorcum—Anal orifice rayed. 
The body is cylindrical. The anal orifice has six very 
unequal rays; the three largest forming the exterior mar- 
gin of the central star. The stomach is simple. The angles 
of the branchial meshes destitute of the papilla. Ovarium 
single, sessile attached to the bottom of the abdomen, and 
