210 ZOOLOGY. 



sac into a small stomach, the intestine bending back on 

 itself, and the vent being near the month. The entire diges- 

 tive canal is immovable, the food being driven through the 

 permanently distended cavity by means of the cilia lining 

 its inner surface. The great posterior blood-sinus surrounds 

 the digestive system on all sides, the nutriment being di- 

 rectly absorbed from its surface and mixed with the blood. 



The nervous system is, in adaptation to its locomotive life, 

 more specialized than in the sessile forms, and highly spe- 

 cialized organs of sight and hearing are present. The heart 

 is a short, complicated organ, lying in the sinus-system. Its 

 action is often reversed ; the reversal of the beats tending 

 to clear the sinuses of the blood-disks overcrowding them. 

 In one species of Salpa Prof. Brooks states that the blood- 

 channels are in all cases sinuses, which are parts of the body- 

 cavity and have no special walls, though in species investi- 

 gated by other writers there are said to be true blood-vessels, 

 lined with epithelium. 



The hermaphroditic, aggregated or chain-salpa differs from 

 the solitary asexual form in being less regularly barrel- 

 shaped, and without the two long posterior appendages of 

 the latter ; in the proportions of the different organs, the 

 two forms are essentially alike. 



The young chain is easily perceived in the solitary indi- 

 viduals in the posterior part of the body, curving around the 

 digestive organs. When first set free from the body of the 

 solitary Salpa, the chain is about half an inch long, and the 

 single, individual Salpae composing it arc about two and a half 

 millimetres in length. They grow very rapidly, and soon 

 reach their full size, when the chains are often a foot or a 

 foot and a half long ; the individuals composing them when 

 fully grown being about two centimetres in length. The 

 chain easily falls apart, and the individuals are capable of 

 living a solitary life, Huxley stating that the chain-individu- 

 als of the species observed by him were generally found soli- 

 tary ; for this reason we should regard the chain-salpse as 

 individuals, not zooids, being capable of leading an inde- 

 pendent existence, and with a structure almost identical with 

 that of the solitary Salpae. 



