180 Miscellaneous. 



The question now arises, "What are the affinities of the curious 

 larva described above ? It has Chaetopod, Brachiopod, and Bryo- 

 zoan features, and may be supposed to resemble the archetype or 

 ancestral form of these three groups. 



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

 hratidina *, 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 Cydopehna, the young of Loxosoma, often- 

 times regarded a Bryozoan. Its closest affinities appear to me to 

 be with Mitraria, a larva which Metschnikoff has already shown to 

 belong to the developmental stages of a Cha^topod annelid. It 

 differs, however, considerably from Mitraria, and its true affinities, 

 whether with Brachiopods or Cha^topods, must be discovered by 

 later investigation. 



Balfour, in his weU known ' Comparative Embryology,' has saga- 

 ciously suggested that Pilidium, a larval form of certain Ncmertean 

 worms, reproduces the larval prototype in the course of its conver- 

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

 still further, 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 Pilidivm 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 pro- 

 bably more or less modified by secondary characters ; but the essen- 

 tial form of the young Pilidium is probably ancestral for several 

 groups of marine animals. 



Following the Pilidium-stage in the groups of Brachiopods, 

 Cha^topods, and Bryozoa is one which we may call the Mitraria- 

 stage. It is thought to be assumed, possibly in a modified form but 

 with certain general features which arc characteristic, by the young 

 of certain genera of each of the three groups mentioned. 



It is the opinion of the author that while the beautiful Mitraria- 

 like laiTa here figured has many secondary characters which are 

 not ancestral for the Bryozoa, Chsetopoda, and Brachiopoda, it also 

 has features which are phylogenetic for the three groups. Con- 

 sidering, then, the Pilidium as a stage following the gastrula, the 

 next stage in these groups may not be unlike the Mitraria. This 

 stage, which may be looked upon as a common one in the three 

 groups named, adds to the gastrula, among other features, the fol- 

 lowing : — 1. An apical tuft of cilia mounted upon an epiblastic 

 thickening ; 2. A mouth surrounded by a ciliated rim ; 3. A pro- 

 tuberance near the mouth from which arise embryonic setoe. 



* It cannot be asserted dogmatically that my new larva is not a Bra- 

 chiopod ; but it diifers essentially from the larval Brachiopods which Lave 

 been described. 



