MUSEUM OF COMPARATIVE ZOOLOGY. 191 



stage," similar in some respects to a metamorphosis which has been described 

 in some Gephyrean worms. The enclosed Nemertean, drawing back its probos- 

 cis out of the pigmented sheath, protrudes it outside the body walls thi'ough an 

 orifice at the lower end of the larva. lu a single specimen (Fig. 15) in which 

 the " critical stage " was observed, this opening was seen below the ring of 

 cilia on the lower end of the larva. After resting a moment, suddenly, either 

 normally or abnormally, the half-protruded proboscis is forced still farther 

 outside the larva, turning the Pilidium in such a way that it is everted, and 

 appears as a shrunken remnant, forming a marked enlargement at the poste- 

 rior end of the body. No part of the "nurse" is unabsorbed, and even the 

 pigmented regions of the amnion described above can be detected in the en- 

 largement which characterizes the posterior extremity of a Nemertean which 

 has just passed through the critical stage. In other species of Pilidiwm a 

 majority of authors declare that the original larva, with its enclosed amnion, 

 lives independentlj^ for some time after the escape of its Nemertean. Such is 

 not the case in a single specimen of P. recurvatum, which was made to pass 

 through the critical stage in the way described above. The body of the larva 

 with the amnion is here absorbed into the posterior end of the body of a grow- 

 ing worm, which it carried in a way not unlike that in which the pluteus bears 

 the young Sea-urchin. The absorption of the larval envelope in Pilidium 

 recurvatum is in reality a true Echinoderm feature, and seems to me one more 

 '.haracteristic pointing to the close affinities of these worms and the Echino- 

 Jerms. 



The form of the worm, after it has passed through the wonderful meta- 

 morphosis which has just been described, is shown in Figs. 11 and 12. It is 

 now no longer free-swimming, as formerly, and, although ciliated over its whole 

 exterior, is slow moving, and immediately sinks to the bottom of the aquarium 

 in which it is confined. The worm just after the metamorphosis from the 

 Pilidium is elongated in shape, pointed at the anterior, and swollen almost 

 globular at the posterior extremity. It is ciliated on its whole external sur- 

 face. The body is semitransparent, and large patches of pigment appear in 

 the enlargement at the posterior end of the body. Cephalic sacs, with ciliated 

 linings, are well marked, but no eye-spots are seen on the head of the Nemer- 

 tean immediately after its escape from the Pilidium. According to Biitschli 

 the fully grown Nemertes of P. gyrans is probably destitute of ocelli. In the 

 few specimens of P. recurvatum which were found at Newport, no ocelli were 

 observed, while the worm was contained in the amnion previous to the critical 

 stage, and it was only later that the two eye-spots were formed. The oldest 

 form which we have obtained of the Nemertean derived from P. recurvatum 

 is yet a long distance structurally speaking from the adult, but yet has so 

 many resemblances to Linens that I have referred it provisionally to this 

 genus. Its general appearance from the dorsal side is shown in Fig. 14. 



Many prominent differences between this and the last stages which have 

 been described are found in the general outlines of the body. The great en- 

 largement at the posterior extremity of the worm just escaped from the larva. 



