464 DISCOVERY REPORTS 



the stream of water without warning. In the ordinary tow-net where the bridles and 

 towing rope precede the entrance a number of the more agile animals may escape 

 capture by darting aside at the approach of the ropes. This absence of obstruction is 

 important with so small an opening. 



To sum up, the general principle of the mechanism may be compared with that used 

 by the animal Amphtoxus {Branchiostoma). The machine has a small opening leading 

 into a larger cavity with an enlarged filtering surface. The size of the entrance can be 

 adjusted and in the first machine, where it was larger than in later ones, there was a 

 grid-work to keep out large objects. Instead of the plankton sieved out being carried 

 off the " netting" by streams of mucus from the endostyle, the netting itself is made to 

 move and carry the plankton with it. Just as the mucus collected in the epibranchial 

 groove of Amphtoxus carries the planktonic food in a continuous stream towards the 

 oesophagus, so the gauze bandings carry the plankton continuously towards the storage 

 chamber. Into this chamber is secreted, by narrow ducts from " a gland-like organ", a 

 fluid: preservative instead of digestive. Only recently have I realized that my "in- 

 vention" is but an imitation of Nature's mechanism. 



THE FIRST MACHINE AND ITS PRINCIPAL DEFECTS 

 Before passing to the latest machine it will be convenient to describe briefly the first 

 machine made, in order that the defects encountered may be put on record and so 

 avoided by possible future workers who may design modifications of the machine. 



External Features 

 The machine as first designed (Type I) is illustrated in Plate I. The main part of 

 the body was cylindrical but tapered in front and behind to openings : the entrance and 

 exit to the water tunnel. The tapered portion in front was shorter than that behind so 

 as to give the body an approximate streamline design. Its total length was 4 ft. z\ in., 

 and the diameter of the cylindrical part \z\ in. The openings were \\ in. in diameter, 

 but the front one could be reduced to 3! in. and 3 in. diameter by screwing on metal 

 collars. The front opening was also provided with a grid of crossing wires 1 in. apart to 

 prevent the entrance of large objects such as fish; at times they cut jellyfish and Pyro- 

 soma into neat cubes. The cylindrical body of the machine was cut in two towards its 

 wider end and hinged below, so as to open and give access to the internal mechanism 

 for adjustment and loading of the spools (Plate I, fig. 2). When closed the two parts 

 were held tightly together below by the hinges and above by a pair of clamping pieces 

 hinged on the forward part and fitted with screws bearing on lugs on the rear part 

 (shown clearly in the plate). The propeller, with adjustable blades, was situated aft 

 within a guard ring, and the propeller shaft passed forward into a gear box packed and 

 kept filled with oil. The cap for replenishing the oil is seen in the photographs at the 

 top of the gear box. The propeller shaft passed through a lignum vitae bearing placed 

 between vertical fins and between two horizontal vanes which could be tilted up or down 

 to act as elevators to adjust the stability of the machine. To the sides of the machine. 



