2940 THE SEA S'QUIRTS OR ASCIDIANS 



so-called atrial cavity, inclosing to a great extent the viscera, and communicating 

 with the exterior by means of the exhalent orifice. The inhalent orifice, or mouth, 

 communicates, on the other hand, directly with the exceedingly large pharynx or 

 branchial chamber, which extends nearly to the hinder end of the body, and is per- 

 forated 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 aw r ay 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 circula- 

 tory 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 fre- 

 quently lying within the loop of the intestine, and discharging into the atrial cavity 

 along side of the vent. A remarkable 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 re- 

 semblance between the adult sea squirts and the lancelets is to be found in the pos- 

 session 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 hermaphro- 

 dite 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 

 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 im- 

 portant 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 development a groove makes its 

 appearance on one surface of one ascidian embryo, the large cells on the side of which 

 grow inward so as to inclose a tube, corresponding to the nerve tube of Vertebrates, 

 beneath which is the notochord. When of an oval shape, and while still contained 



