424 MORGAA r . [Vol. V. 



The larva soon assumes the shape of the adult Balanoglossus. 

 The oldest larva I obtained was kept seventy hours after it was 

 caught in the tow net. In Fig. 9, PL XXIV., is shown the larva 

 from dorsal view, and as drawn from a preserved specimen. Agas- 

 siz has given a figure of the living larva at about this stage. The 

 larva is divided into three regions, — the proboscis, the collar, and 

 the body proper. The circular band of cilia is still retained and 

 has moved far back towards the anal end of the larva, owing to 

 the increase in length of the collar and the region just behind it. 



The internal organs as shown by sections exhibit in many 

 points the transition between the Tornaria and the adult. The 

 series of Figs. 58-66, PI. XXVIII., are from transverse sec- 

 tions of this larva, and illustrate the most important changes 

 which have taken place. Their planes with reference to Fig. 9, 

 PI. XXIV., are shown by the lines 58-66. 



We may first examine the structures shown in Fig. 58, which 

 is a transverse section through the anterior collar region. The 

 ectoderm is very thick and ciliated and is filled with gland cells 

 staining deeply. The inner layer of the ectoderm is formed of 

 a nerve-fibre layer, but in the ectoderm in the dorsal line — above 

 the nerve chord — it is not present. The nervous system (N) 

 is completely separated from the ectoderm and is partially 

 arched over by the body cavities. The chord is separated into 

 two parts by a narrow crescentic lumen. The cells above this 

 are arranged in a single row and have no nerve-fibre layer. 

 The cells below the central cavity are very numerous and have 

 a well-developed nerve-fibre layer in the lower part of the chord. 

 The digestive tract — oesophagus in this section — shows a ten- 

 dency to division into two parts by lateral constrictions in its 

 walls, forming a larger lower part and a smaller upper, the latter 

 is directly continuous into the notochord farther forward. In the 

 corners of the folds are two thickened chitin-like rods. The 

 space between the walls of the digestive tract and the ectoderm 

 is filled up by the body cavities of the collar (be 2 ) (first paired 

 cavities). Each sac has a splanchnic and somic wall, which 

 are applied to the digestive tract and body walls respectively. 

 (In this section the body cavities have to some extent shrunken 

 away from the ectoderm and digestive tract.) The large cen- 

 tral space of the sacs is filled with a branching menchymatous- 

 like tissue. Between the nerve chord and the digestive tract 



