THE ENTEROPNEUSTA. 35 



essentially the same as in B. minutus. In the anterior 

 region of the notochord of B. Brooksii there are hardly any 

 nuclei at all. I hope to publish figures illustrative of the later 

 development and anatomy of the other parts at no distant date. 



Methods of Investigation. 



From the characters of the unfertilized egg of B. Kowa- 

 levskii it was extremely improbable that the earliest stages 

 of development could be passed anywhere else but in the mud 

 which the parents inhabit. Though the examination of 

 Agassiz had failed in the attempt to find any but adult 

 Balanoglossus in this situation it seemed worth repeating. 

 Accordingly a large quantity of mud inhabited by Balanoglossus 

 was placed in a glass vessel of water and worked up, avoiding 

 rotatory currents, until the whole was in suspension. A number 

 of Balanoglossus which had previously been minced very finely 

 were then thrown in and the whole was then left to settle for a 

 few minutes. I then siphoned off the water and lighter parti- 

 cles in suspension, which consisted chiefly of vegetable debi'is, 

 stopping the siphon when the layer of chopped Balanoglossus 

 was reached, which could easily be seen by the bright orange 

 colour of the fragments. This portion was drawn off and 

 examined separately. It was found to contain great numbers 

 of larvae and embryos of Balanoglossus, minute Nemertines, 

 free Nematodes, &c. In this manner all the animals 

 living in several hundredweight of mud may, in an hour or 

 two, be collected into about a pint of water and sorted with a 

 simple microscope. This was generally performed by rotating 

 a little of the water in a shallow saucer with a slight peripheral 

 groove. The larvae then all lie in the groove which may be 

 passed under the lens by rotation. After a little practice it 

 becomes unnecessary to discolour the water with fragments of 

 the animal required, as of course the right layer can easily be 

 detected by the size and character of the particles composing 

 it. It appeared to be worth mentioning this mode of obtaining 

 mud larvae as its application on a large scale does not seem to 

 be generally employed. It should be remarked that small 



