SEA-SQUIRTS. 563 



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 exhalent 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 Chorclates, 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. .. . ... 



through a free-swimming 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 



