SEA-SQ UIR TS. 
5 6 3 
large pharynx or branchial chamber, which extends nearly to the hinder end of 
the body, and is perforated by a vast number of gill-openings, through which the 
water taken in at the mouth passes into the atrial chamber. Instead of passing 
directly into the latter chamber with the water, the food is caught up in a mass of 
slime, and carried round the base of the mouth-tube until it reaches the entrance 
to the oesophagus, which lies near the hinder end of the dorsal surface of the 
branchial chamber. Hence it passes into the stomach, and along the intestine, 
which forms a U-shaped curve turned away from the dorsal aspect; the vent 
opening on the same aspect into the atrial cavity below the exlialent orifice. With 
regard to the nervous and circulatory system, it will suffice to say that there is a 
large nerve-ganglion embedded in the tissue of the inner tunic, and lying on the 
dorsal surface of the body between the inhalent and exhalent orifices; and true 
blood-vessels are wanting, the blood merely flowing through a series of spaces in 
the muscles and other tissues of the body and between the viscera, and the heart 
forming a dilated tube. Unlike the higher Chordates, all the ascidians are 
hermaphrodite; the reproductive organs frequently lying within the loop of the 
intestine, and discharging into the atrial cavity alongside of the vent. A remark¬ 
able physiological feature of the group is to be found in the periodical reversal 
of the action of the heart; the blood being driven for a certain time in one 
direction, after which the heart makes a short pause, and then propels it in an 
opposite course. 
In addition to certain other structural features, into the consideration of which 
it would be impossible to enter in a work of the present nature, the essential 
resemblance between the adult sea-squirts and the lancelets is to be found in the 
possession by both of a pharynx perforated by a large number of gill-openings, 
which convert it into a branchial chamber, opening into an atrial cavity instead of 
directly to the exterior. Several of the differences between the two, such as the 
hermaphrodite reproduction and the bent intestine of the sea-squirts, are probably 
due to their sessile habits, since such features are characteristic of most fixed 
organisms. Other points of difference are to be found in the absence of segmentation, 
and the want of a dorsal nerve-tube and notochord in the adult ascidian, although, 
as we shall see, a remnant of the latter exists in the tail during the larval condition. 
All ascidians, whether fixed or free in the adult condition, go 
Development. a f ree _ sw imming larval stage, during a part of which they 
develop a tail containing a notochord and nerve-tube; and as this feature is all 
important from a morphological point of view, it must be mentioned here, although 
necessarily in a very brief manner. Generally the larval condition lasts but a 
short time; and this may be the reason for the development of the tail, as a 
powerful swimming organ would seem to be essential in order to enable the 
creature to reach a spot suitable for its permanent existence. During its develop¬ 
ment a groove makes its appearance on one surface of the ascidian embryo, the 
large cells on the side of which grow inwards so as to enclose a tube, corresponding 
to the nerve-tube of Vertebrates, beneath which is the notochord. When of an 
oval shape, and while still contained in its investing membrane, the embryo assumes 
a ventral curvature, and at the same time produces a long tapering tail, which 
eventually becomes coiled round it. In addition to certain other structures, this 
