MUSEUM OF COMPARATIVE ZOOLOGY. 193 



Holothurians the body girt by several parallel belts of cilia. One only of 

 tbese rings of large cilia remains unchanged in Tornaria and in Pilidium recur- 

 vatum ; but in the former genus two others, very much modified in position 

 and never parallel, form the loop-like bands between which the mouth opens. 

 These bands, quite simple, as I shall later show in the young Tornaria, have a 

 very tortuous course later in their career, but never attain the complexity 

 which marks the course of homologous bands on the young of our common 

 Starfish or Sea-urchin. Much greater than its resemblance to the young Echi- 

 noderm is the likeness of our new Pilidium to the well-known Tornaria. 



On the same plate wath my figures of the larvae of P. recnrvahim are 

 introduced for a comparison two illustrations of very young stages in the 

 growth of Tornaria (Balanoglossus). These are still younger than any larvas 

 which are yet known of our American Tornaria, and present many very inter- 

 esting features. The closeness of the relationship between them and the 

 younger members of the series of Pilidium which they accompany is not the 

 least interesting of the many comparisons which they suggest. 



In the youngest (Fig. 16) we have a Tornaria of an irregular pear-shaped 

 form, with well-marked oesophagus, stomach, and intestine. A mouth opens 

 on one side of the body and an anus is found at its lower pole. The external 

 surface of the body is crossed by two simple ciliated bands. These have a 

 common union at the upper pole of the larva, but a very divergent course on 

 its external surface. The shorter of these ciliated bands forms a loop varying 

 slightly from the form of a ring, which extends from the upper pole nearly to 

 the equator, but never into the lower hemisphere. The larger band has a 

 more tortuous course than the other, which it resembles in its loop-like form. 

 It is much longer, and extends into the lower hemisphere almost to the lower 

 pole. It meets in its course the smaller band only at one point, which is at the 

 upper pole of the embryo. The mouth opening of the young Tornaria lies on 

 its equator under the eaves of a projecting upper hemisphere, and between 

 these two ciliated bands. At the common junction of the two ciliated bands 

 is found a pair of eye-spots, above which rises a small tuft of cilia. The Tor- 

 naria swims with this region uppermost in the water. From that part of the 

 larva upon which these ocelli are borne, extending internally to the neighbor- 

 hood of the union of oesophagus and stomach, passes a muscular thread very 

 similar to like threads already mentioned in Pilidium. An unpaired tul^e 

 extends from the point of union of the oesophagus and stomach, on its dorsal 

 side, to the middle of the dorsal flexure, opening externally by a "dorsal 

 pore" about diametrically opposite the mouth. The sac or enlargement of 

 this tube at its inner terminus has not yet reached any great size. 



In this youngest Tornaria there are, as appears also in Miiller's original 

 description of Tornaria, no ring of large cilia near the anal pole and no lateral 

 bodies (" lateral plates," "lappets," A. Agassiz) by the side of the stomach, 

 such as we find in the older Tornarice. All theoretical questions which 

 consider a comparison of these last bodies to the water-tubes of the Star- 

 fish larvee must take cognizance of the fact that the median water-tube, which 



VOL. XI. — NO. 9. 13 



