Phylum Mollusca

Coelom Mesenteries

- organs are suspended by mesenteries

- blood vessels are located in mesenteries

- digestive tract more specialized

- evolution of larger more complex forms

Basic Characteristics of Phylum:

1.a. Head

Radula within mouth

1b. Foot -

2. Visceral Mass

Body and organs

Mantle -

3. Respiratory System

two ctenidia (gills) - leaf-like filaments.

Counter Current Exchange Mechanism

(High O² content) (Low O² content)

Blood flow-------------------------------->

<--------------------------------------Water flow

(Low O² content) (High O² content)

4. Circulatory System

Open circulatory system =

**exception: the cephalopods with a closed circulatory system

5. Excretory System - Pair of kidneys

Metanephridia = inner tube opens into the coelom by a nephrostome

[similar to protonephridia but they work as an organ (multcelluar level) while the protonephridia were based on flame cells and were thus cellular in their basic structure.]

6. Nervous System -

7. Digestive System

complex.

- The gills trap food particles brought into mantle cavity

- Cilia move food particles to gill margins then to mouth

- leaf-like labial palps on each side of mouth sort and filter food

- food enters an esophagus and then to a stomach.

- A crystalline style projects into stomach and secretes enzymes for digestion. The style also rotates and causes abrasion of food.

- the finer particles enter the intestine whereas larger particles enter the digestive gland for further digestion. Intestine empties through anus.

Direct metamorphosis to juvenile

8. Reproduction (Chitons) primitive condition

most dioecious trochophore

veliger stage

(Gastropods and bivalves) derived condition

Or No free swimming larval stage and egg hatches into a juvenile (Cephalopods)

9. Shell

a. Periostracum -

b. Prismatic layer -

c. Nacreous Layer -

Nacre forms pearls

Class Polyplacophora

"Many plates" Chitons

intertidal zone to depths of over 12,000 feet but most abundant on temperate rocky shores

Class Scaphopoda

Tusk Shells or Tooth Shells

tubular shell open at both ends. Mantle wraps around body to form tube.

Foot extends out large end of tube

Gas exchange in mantle - no gills

Class Gastropoda

Snails, slugs, whelks, conchs, sea slugs, sea hares, and limpets

Univalve shell

Operculum -

Torsion = phenomenon whereby the mantle cavity, which was originally posterior, is moved to the anterior of the body. Occurs during veliger stage.

Before torsion -

After tortsion -

Only in gastropods.

Problem?? Waste expelled above head!

Why torsion?? Sense organs now face in direction of travel.

Coiling =

A. Planospiral (Ancestral condition) - All whorls in single plane.

B. Conispiral - eliminated compaction problem. Each succeeding whorl at side of preceding one. But unbalanced with whorls hanging on one side.

C. Shell shifted up and posterior for better weight distribution. Now weight predominately on one side pressing on mantle cavity. Result = loss of gill and kidney on right side.

Gastropods feed using a radula. Most herbivores. Others scavengers.

Like mollusc in general use ctenidia for respiration. But pulmonates use vascular area on mantle as a lung.

Class Bivalvia - hatchet-footed animals

mussels, clams, scallops, oysters, and shipworms

No head; no radula

Umbo oldest part of shell

Class Cephalopoda

squids, octopuses, nautiluses, devilfish, and cuttlefish

Nautilus shell looks like a gastropod shell but quite different. Divided by transverse septa and the animal lives in the last chamber.

Siphuncle - cord of living tissue that connects the chambers; extends from visceral mass.

Chambers are gas filled which keeps shell upright.

Squids

Octopus

Squid swim by forcing water from mantle cavity through a ventral funnel = jet propulsion

Active life style

In bivalves, cilia draws water over gills and mantle

In cephalopods, high O2 needs - use muscles to draw in water

Closed circulatory system - capillaries conduct blood through gill filaments. Accessory or branchial hearts located at base of gills to increase pressure.

Brain largest of any invertebrate - millions of nerve cells

Complex eye - cornea, lens, chambers and retina.

Capable of observational learning

Use movements and color change to communicate

Ink sac - secretes sepia

Internal fertilization - dioecious; males use hectocotylus.