64 ON A TORNARIA FOUND IN BEITISH SEAS. 



measuring not more tlian '33 mm. in length. In the same surface 

 net I found a few specimens of a younger larva which is identical 

 with Johannes Miiller's figure of the youngest Tornaria found by 

 him. One of these larva3 is shown in fig. 1. It has a pre -oral 

 ciliated band and a longitudinal band, but the circumanal band is 

 not developed. The anus is terminal and the mouth opens on the 

 ventral surface. A longitudinal section of this larva is shown in 

 fig. 2. The segmentation cavity is large, and in its anterior portion 

 are several scattered amoeboid cells, which will form the walls of the 

 anterior body-cavity. The mouth leads into a stomodseum which 

 is not yet in communication with the gut. The gut is divided into 

 two regions, an anterior mid-gut and a posterior hind-gut, the latter 

 opening to the exterior by the anus. The opening of the anus is 

 shown in fig. 3. It seems probable from the condition of the larva 

 that the posterior division of the gut is not a proctodsBum, as might 

 be supposed from an examination of the perfect Tornaria, and that 

 the blastopore persists as the anus without being pushed further 

 inwards by a secondary invagination of ectoderm, i. e. a proctodgeum. 



The next stages observed have all the characters of a Tornaria. 

 The circumanal ciliated ring is now fully developed, but has not the 

 same importance that it acquires at a later stage. The pre-oral and 

 longitudinal bands retain the same arrangement, but are more 

 marked than they were in the earliest stage (fig. 4). The gut is 

 increased in size and is more distinctly differentiated into a mid-gut 

 and a hind-gut. The anterior body-cavity is formed, probably from 

 the amoeboid cells described in the earlier larva, and is connected by 

 a muscular thread with the now conspicuous apical sense organ. It 

 opens on the dorsal surface by a pore, which eventually becomes, as 

 Spengel* has shown, the proboscis pore of the adult. Anticipating 

 its fate I shall refer to this pore as the proboscis pore. Fig. 12 

 shows a longitudinal section through a Tornaria of this age. It can 

 be seen that the oesophagus opens into the gut, the point of junction 

 between the two being sharply defined. The cells of the oesophagus 

 are columnar and richly ciliated. In larvae of this age the opening 

 from the mid -gut into the hind-gut is very small and difiicult to 

 find in specimens contracted by reagents. At first I was led to 

 believe that no communication between the two exists at this stage, 

 but further examination showed me that an opening exists as shown 

 in fig. 5. No traces of the so-called heart (the proboscis sac of 

 Bateson) nor of the collar- and body-cavities are to be found at this 

 stage. The third stage shown in fig. 13 represents the perfect 

 Tornaria. The lai'va has increased greatly in size, the specimen 

 from which the figure was drawn was as much as a millimetre in 



* Bau u. Entwicklung von Balancglossus, Tagebl. d. Naturf. Vers. Miinchen, 1877. 



