2948 THE SEA SQUIRTS OR ASCIDIANS 



on the lower surface of the body. Pelagic in habit, and transparent in structure, 

 salpae have been not inaptly compared to a barrel with both ends knocked out; and 

 really consist of little more than a huge pharynx, swimming through the water, and 

 taking in large mouthfuls of the same at each contraction of its muscles. Through 

 the hollow, to below the hinder aperture, runs obliquely a rod-like gill (d) from 

 above the mouth, although this is too narrow to interfere with the free flow of the 

 water; while the lower surface of the interior of the creature is furnished with a 

 ciliated slime-secreting band, corresponding to the structure known in other ascid- 

 ians and the lancelet as the endostyle. It may here be well to mention that in the 

 lancelet the structure in question is an elongated gland situated at the base of the 

 pharynx, and against which the ends of the gill bars abut. The only part of the 

 salpa that is not transparent is the thick mass of viscera (e, c) at the hinder end of 

 the body; while the muscular bands, by the contraction of which the water is driven 

 through the barrel, may be compared to the hoops of the latter. Externally the 

 whole animal is invested with a thick, tough, transparent tunic; and in some forms 

 there are two tail-like appendages to the hinder end of the body. Such is the 

 structure of a salpa; but there are two generations in the life of these creatures, 

 namely, the simple form, and the chain-like or aggregate form; the first being shown 

 in the upper, and the second in the lower figure of the plate facing p. 2938. It \vill 

 be observed that in the chain the individuals are attached to one another by their 

 upper and lower surfaces, and thus have these two apertures free; and when taken 

 from the water the whole chain, which is several feet in length, can be easily 

 resolved into its component units. The specimen represented in the figure on p. 2947 

 is one of these detached units from a chain, the projection marked g being for the 

 purpose of attachment to the neighboring individual. Although extremely inter- 

 esting and curious, the whole history of the development of salpae is so complicated 

 that it is almost impossible to explain it fully in a popular work. It maybe stated, 

 however, that the solitary salpa is born from an egg carried within the body of one 

 unit of the aggregate form, the embryo being nourished by means of a placenta 

 from the blood of the parent. On the other hand, the chain salpae are produced 

 asexually by budding from a stolon within the body of the solitary form. In the 

 chain salpa the eggs arise, however, at an exceedingly early period of its develop- 

 ment, with the curious result that three generations are present at one time in a 

 single individual. Thus a solitary salpa has within it the buds of an aggregate 

 salpa, the units of which may each contain eggs which will ultimately develop into 

 the next solitary form. And, as a matter of fact, in a solitary salpa the germ cells 

 of the embryo of the next solitary form are actually visible before the development 

 of the stolon which is to give rise to the chain form. As the stolon forms in the 

 body of the latter, it includes within it the mass of germinal cells; and while the 

 former elongates to form the chain of units, the mass of germ cells likewise lengthens, 

 with the result that a single egg cell is shut off in each unit of the chain. Simple 

 salpae vary in size from a quarter of an inch to upward of eight inches; and in 

 some parts of the ocean surface are met with in incalculable swarms. Although 

 more abundant in tropical than in the cooler seas, their northward range extends 

 beyond Scotland and Norway, while to the south they have been taken below the 



