200 ASCIDIANS AND LANCELETS 



as the case may be, of the aggregate forms ; and that the chained 

 Salpae do not produce chained Salpae, but solitary individuals, 

 which in turn produce chained Salpas. 1 The sexes are united, the 

 ovary and testis being closely connected ; but in many cases the 

 two sets of reproductive organs are not matured at the same time, 

 so that self-fertilisation is prevented. 



Brief mention must be made of the curious and interesting 

 Balanoglossus, considered to be related to the lower Vertebrates. 

 It is a soft-bodied, worm-like animal, which occurs in the sea, bur- 

 rowing in sand and mud. Its body is divisible into three parts 

 externally : (i) the large club-shaped hollow frontal organ, the 

 proboscis ; (2) the prominent fold immediately behind the pro- 

 boscis, called the collar ; and (3) the long and nearly cylindrical 

 trunk. Within the body the most striking anatomical points are 

 the presence of a notochord and gill-slits which are developed as 

 in the Vertebrates, as paired outgrowths of the alimentary canal. 

 The sexes are separate. The progress of development varies in 

 different species, being comparatively direct in some ; while in 

 others there is a regular metamorphosis. In the latter case the 

 larval form, termed a Tonaria, is so like an Echinoderm larva that 

 Johann Miiller, who first described and named it, considered 

 it to be the larva of a starfish. It is provided with a pair of 

 ciliated bands and an independent circlet of strong cilia at the 

 posterior end. 



We now come to a small group of marine animals, of which the 

 best-known form is the Lancelet or Amphioxus (Branchiostoma 

 [Amphioxus] lanceolatum), a semi-transparent, slightly iridescent 

 little creature, pointed at both ends and about 2 or 3 inches long. 

 A very noticeable external feature is a series of V-shaped markings 

 along the body, which are due to cross-partitions of connective 

 tissue that divide the longitudinal muscles of the sides of the body 

 into successive blocks or segments. The mouth is just behind the 

 anterior tip of the body, and is surrounded by delicate cirri, which 

 by their movements waft in food particles. The mouth leads into 

 the largest section of the enteric canal, the pharynx, a high, com- 

 pressed chamber, the walls of which are perforated by more than 

 a hundred pairs of narrow, oblique clefts, the gill-slits or branchial 



1 The above brief outline of the chief characteristics of the Tunicates has been 

 8 ndensed from the writings of Dr. Henry Woodward. 



