Anaspidea - gizzard
Drawing of the gizzard region of Aplysia, opened to show cuticular plates. [Modified from an original by G. Cuvier (1803)]
The digestive system of the Anaspidea is of very similar design from the primitive Akera through to the sea hares. They have a unique oesophageal gizzard which distinguishes them from other opisthobranchs. There is a muscular buccal bulb (bb), opening at the mouth, and it contains the broad rasping radular ribbon used to break off pieces of algae. A pair of long salivary glands (sg) open into it. The oesophagus has become a large extensible sac or crop (cr) in which pieces of algal food are stored before being crushed in the gizzard, which is broken into 3 regions (A-B-C). The first (A) is unarmed and presumably is just an anterior holding sac for partially digested food forced down from the crop. The second part of the gizzard (B) has a basal layer of circular muscle encircling the gizzard, and is lined with large chitinous pyramidal crushing plates which reduce the algal plates to fine pieces and a slurry of algal 'juice'. The third region (C) is lined with find recurved chitinous hooks which filter the crushed slurry of food, preventing large particles from moving further down the gut.
The stomach (st) is a very small chamber into which the ducts (D) of the digestive gland open. Also opening into the stomach are the caecum (F) and the intestine (E). Then stomach and caecum are lined with ciliated ridges and there is obviously a very complicated sorting process going on in the stomach, separating edible and inedible particles from the gizzard, and faecal and excretory products from the digestive gland. The caecum seems to be where the faecal pellets are formed.
In summary, the gizzard plates are of two sorts. A band of large crushing plates and then a region of relatively fine hooks packed close together to form a filter chamber.Authorship details
Rudman, W.B., 2004 (December 22) Anaspidea - gizzard. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/anasgizz