164 The Microscope. ^ 



this epiblastic thickening. The digrestive system of our larva is 

 very simple, and its yellow walls are readily seen through the sides 

 of the body. It consists of a long, tubular oesophagus, the inner 

 wall of which is richly ciliated, opening into an elongated stomach, 

 simple and without cilia.* The mouth lies just inside the ciliated 

 rim or belt, and is separated from the stomach by the globular body, 

 at the base of the spine-bearing protuberance on the lower pole 

 of the larva. 



The larva is, when expanded, from .15 to .2 mm. in diameter. 



Only a single stage in the growth of this lai*va was found, and 

 consequently its adult form is unknown. 



The question now arises, what are the affinities of the curious 

 larva described above. It has chsetopod, brachiopod and bryozoan 

 features, and may be supposed to resemble the architype or ancestral 

 form of these three groups. 



I was at first led to regard it the young of the genus terebra- 

 tulina,f a brachiopod common in the Bay of Fundy. It difFers 

 however, very considerably from any figure of a brachiopod which I 

 have ever seen, although in some features it recalls Argiope. It also 

 resembles somewhat Cyclopelma, the young of Loxosoma often-times 

 regarded a bryozoan. Its closest affinities apppears to me to be with 

 Mitraria, a larva which Metschnikoff has already shown to belong to 

 the developmental stages of a chsetopod annelid. It difFers however 

 considerably from Mitraria and its true affinites, whether with 

 brachiopods or chsetopods, must be discovered by later investigation. 



Balfour, in his well known Comparative Embryology, has saga- 

 ciously suggested that Pilidium, a larval form of certain nemertean 

 worms, reproduces the larval prototype in the course of its conversion 

 into a bilateral form. Other naturalists have carried the idea still 

 fuiiher, and find the Pilidium to represent a definite stage in the 

 development of several groups of marine larvae. While I cannot 

 subscribe to many of the statements made by the several naturalists 

 who have written on this subject, it seems to me not improbable that 

 Balfour's interpretation of the signification of the Pilidium as a 

 definite ancestral stage may be considerably amplified, and that the 

 Pilidium or a Pilidium-like larva may be recognized in other groups 

 •than that of the nemerteans. The well-developed Pilidium is 

 probably more or less modified by secondary characters, but the 

 essential form of the young Pilidium is probably ancestral for 

 several groups of marine animals. 



* No external opening of the stomach through an intestine, was observed. 

 + It cannot be asserted dogmatically that my new larva is not a brachiopod; it differs 

 essentially from the larval brachiopod which ha\e been described. 



