VOL. III.] Balanoglossus. 195 



bryology of Echinoderms, and among the larvse of the various 

 groups of these animals that he collected with his tow net in the 

 Mediterranean Sea was this which he named Tornaria from the fact 

 that it constantly rotates about its long axis as it progresses through 

 the water. He thought it was probably the larva of some Echino- 

 derm and finally, after studying as many stages as he was ever able 

 to find, decided it to be a Holothurian. Afterward several zoologists 

 collected and described the same larva and were deceived as its 

 original discoverer had been till finally, in 1869, Metschnikoff, " a 

 Russian zoologist, was fortunate enough to see the Tornaria so far 

 transformed into the Balanoglossus as to be able to recognize its 

 true nature. The adult Balanoglossus had been well known for a 

 long time. But although it was now soon established beyond a 

 doubt that Tornaria is the larva of Balanoglossus and not of an 

 Echinoderm its close resemblance to the larva of the latter, particu- 

 larly to Auricularia, the larva of the Holothurian, was recognized by 

 all who studied it. And I may here add that the advance of knowl- 

 edge of both the Echinoderm larva and of Tornaria, even to the 

 present moment, has only served to increase the belief in the minds 

 of many morphologists that there is an actual genetic relationship 

 between the two forms. 



Figures 4, 5, 6 and 7 represent the Tornaria in several stages of 

 its development. Figure 7 represents as early a stage as has ever 

 been seen, the larvse having always been captured after they have 

 escaped from the eg^ and betaken themselves to their free swim- 

 ming life. They are very transparent and at this stage very small, 

 the specimen here figured being between .2 and .3 of a millimeter 

 in length — barely large enough to be visible to the unaided eye, 

 excepting it be accustomed to seeking such objects. From its 

 extreme transparency the internal organs can be easily seen in the 

 living animal. 



On the surface are several thickened bands bearing cilia. In the 

 smallest larvae the course of these bands is comparatively simple, 

 as is shown in Fig. 'j, c. b.' Were the opposite side of the larva 

 to be seen, two more bands would be found in corresponding 

 positions. 



'■* E. Metschnikoff. Untersuchungen iiber die Metamorphose einiger Seethiere. 

 I. Ueber Tornaria. Zeit?chr. f. Wiss. ZooL, Bd. xx, 1870. 



