EMBRYONIC DEVELOPMENT. 



place at two definite points. The anterior thickening which occurs 

 at the apical end of the embryo (Figs. 3, 4 B) may be regarded as 

 the homologue of the neural plate of other larvae, and yields the 

 ganglion known as the brain,* which here, throughout life, retains its 

 original epidermal connection. A second thickening of the ectoderm 

 appears behind the mouth as a semicircular, ciliated swelling. From 

 this, which may be regarded as the equivalent of the post-oral ciliated 

 ring, the row of larval tentacles develops (Figs. 3, 4 B), and also 

 the nerve -strand that runs along their points of insertion. The 

 tentacles begin to appear early as outgrowths of the body-wall; 

 they rapidly increase in number, fresh pairs being added dorsally 

 (Figs. 3, 4 B). If this row of tentacles is to be traced back to a 

 transformation of a post-oral ciliated ring, we may regard a strongly 

 ciliated swelling appearing at the edge of the pre-oral lobe as the 

 pre-oral ciliated ring. The posterior part of the body which carries 

 the dorsally displaced anal aperture now grows out into a large 

 cone, the end of which is surrounded by a circum-anal ciliated ring 

 (Figs. 3, 4 C and D). Three body-regions can be distinguished in 

 the larva, which must now be called the Actinotrocha (Figs. 3, 4 B 

 and C) — (1) the pre-oral lobe; (2) the post-oral section which carries 

 the crown of tentacles and covers the posterior part of the body 

 like an apron ; and (3) the posterior or anal section. 



The changes just described, which lead to the development of 

 the Actinotrocha, take place after the commencement of free larval 

 life. The youngest larva, just hatched from the egg (Fig. 4 A), is 

 still without the crown of tentacles, two small projections lying near 

 the anus being the only indications of the tentacle-rudiments, f 



II. Metamorphosis. 

 The Phoronis larva (Figs. 3 and 4 B), the shape of which has 

 been described above, was discovered by Joh. Muller and described 

 as Actinotrocha branchiata, and was afterwards more carefully 

 examined by Wagener, Gegenbaur, and others. Krohn (No. 7) and 

 Schneider (No. 10) investigated the metamorphosis which led to 

 the production of a Gephyrean-like form, Kowalevsky (No. 6) being 



* The pre-oral lobe, together with the brain of the larva, is, as we shall see 

 below, thrown off during metamorphosis. Since, however, these parts are 

 regenerated later, we may still theoretically trace them back to the corre- 

 sponding parts of the larva. 



t [For the structure of the Actinotrocha, see Masterman's recent work 

 (No. II.) on this larval form. These observations, if confirmed, would show 

 that Phoronis is directly related to Balanoglossus, Cephalodiscus, and Rhabdo- 

 pleura. — Ed.] 



