CHAPTER 14 



I. HEMICHORDA 



This Class includes Balanoglossus (Fig. 123, Chap. 13, p. 169), a burrowing, 

 marine, worm-like animal of which there are several species, varying in 

 length from 2-3 cm. to i\ metres. No eyes are present in the adult 

 animal, but in the early stages of its development it passes through a stage 

 which is termed the " tornaria " (Fig. 125), which resembles in certain 

 respects the trochophore stage of certain echinoderms, rotifers, annelids, 



Fig. 125. — Larval form (Tornaria) of Balanoglossus showing the Eye- 

 Spots on the Apical Plate. (After Spengel.) 



Annulata, and Brachiopoda, more especially in the presence of an apical 

 plate, bearing a vertical tuft of cilia and eye-spots. The development of 

 Balanoglossus has been described by Spengel, Heider, and Morgan. The 

 last author describes the two eye-cups on the apical plate of the 

 tornaria as being anterior and posterior in position (Fig. 124, A, Chap. 13, 

 p. 170). They consist of a single row of elongated ectodermal cells 

 which are clear and converge towards the centre of the pit ; each is 

 surrounded by a mass of pigment which lies on the outer surface and 

 between the two cups (Fig. 124, B, Chap. 13, p. 170) ; at the base of 

 the plate is a thick layer of nerve-fibres. At a latter stage the apical 



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